1 Introduction

The role of domestic courts in the development of rules of international law is an area of research that has received increased scholarly interest in the past decade. Within the formal framework of sources, domestic courts can contribute to the development of international law in broadly three ways: as an expression of state practice or opinio juris for the purpose of customary international law (CIL),Footnote ¹ as a contribution to general principles of law,Footnote ² or as relevant subsequent practice for the purposes of treaty interpretation.Footnote ³ Moreover, scholars have also identified a role for domestic courts beyond the framework of sources, pointing to further contributions of domestic courts to the development of international law. For instance, using the analytical lens of ‘domestic courts as agents of development of international law’ a symposium hosted by the Leiden Journal of International Law demonstrated that while domestic courts may have a limited impact on the development of international law within the regime of sources, they can still exercise an informal influence; particularly so if their pronouncements are taken up and validated or endorsed by other actors.Footnote ⁴ Similarly, the analytical framework of a ‘cycle of contestation and deference’ tells us that contestations by domestic courts in cases where they engage with international law can provoke an international reaction or adjustment of the law in response to the contestation.Footnote ⁵

This chapter examines the contribution that domestic courts may have in the development of rules or guidelines for the interpretation of CIL. The examination is motivated by three considerations. Firstly, unlike in the case of treaties whose interpretation is guided by Article 31 of the Vienna Convention on the Law of Treaties (VCLT) and its customary counterparts, presently we do not have clear rules or guidelines for the interpretation of CIL. In fact, as other chapters in this volume demonstrate, legal scholarship is currently still discussing whether custom as a source of law can be subject to interpretation, and if so, what are apposite methods for its interpretation. While at present little is certain, it has been argued persuasively that custom and treaties cannot always be subject to the same methods of interpretation.Footnote ⁶ Thus, we cannot simply transplant the methodology of treaty interpretation onto custom wholesale, and it might even be the case that custom requires a methodology of its own. Secondly, scholarship on the role of domestic courts in the development of international law has persuasively demonstrated that domestic courts can contribute to international law both formally and informally, especially in areas where there are lacunae or the law is yet to be developed. Thus, the practice of domestic courts with respect to the interpretation of custom may prove a valuable source in our study and understanding of this developing field. Finally, by turning to domestic courts we open the door to a wealth of cases which can provide us with examples and insight into the interpretation of custom. Depending on the legal system in place, domestic courts may be faced with the task of interpreting not only CIL but also domestic custom. Thus, domestic courts may be uniquely positioned to provide insight into the methodology of interpreting custom as a source of law.

With these three considerations in mind, the chapter poses the question: how can interpretive methodologies employed by domestic courts inform the development of rules or guidelines for the interpretation of CIL? The chapter is organised along three substantive sections. Section 2 provides an overview of the current academic discourse with respect to CIL interpretation, and briefly introduces the interpretation of CIL as conceptualised by this chapter. Section 3 turns to the contribution of domestic courts to the development of international law, and maps the existing scholarship on the topic. Section 4 contains the operative contribution of the chapter, and begins with an overview of five domestic cases which contain examples of domestic courts interpreting customary law. It then provides some preliminary observations organised along two lines of inquiry: (i) how can we learn from domestic interpretive practices? and (ii) why should we learn from them? The observations provided in this chapter are part of the author’s ongoing doctoral research focused on the interpretability of CIL and the role of domestic courts in this process. In light of this, the findings presented in it will evolve and be updated with further research.

Before continuing with the chapter, a point of terminology is in order. This chapter uses the terms ‘rules’ and ‘guidelines’ for CIL interpretation broadly and interchangeably. This is because currently there is no set terminology which denotes the parameters according to which CIL is interpreted by relevant actors. One of the main objectives of the TRICI-Law project (of which the present author is a member) is to demonstrate the interpretability of CIL and to identify the parameters which guide the process. Therefore, the chapter presently does not take a position on the nature of these parameters, and the jury is still out on the final appropriate terminology.

2 CIL Interpretation

Before delving into an analysis of the ways in which international law may learn from domestic courts’ practice for the purpose of CIL interpretation, a few paragraphs must be dedicated to the interpretability of CIL and the current scholarly debates surrounding it. A detailed discussion of the interpretation of CIL is beyond the scope of this chapter and is addressed more elaborately elsewhere in this volume.Footnote ⁷ This section is only intended to briefly map the current state of the scholarly discourse, and to show the reader what is the thing that we speak of when we speak of CIL interpretation throughout the chapter.

Unlike treaties, whose interpretation is guided by the VCLT and its customary counterparts, CIL’s interpretation remains a mercurial process whose functioning is both questioned and unregulated. Claims against the interpretability of CIL are broadly organised along two lines: firstly, it is argued that CIL’s unwritten character excludes the need for its interpretation. Here, the argument is that even though language is necessary to communicate the content of customary rules, expression through language is not an indispensable element of CIL rules. Тhis irrelevance of linguistic expression excludes interpretation as a necessary operation in order to apply CIL rules.Footnote ⁸ Secondly, it is argued that CIL cannot be subject to interpretation because if an attempt is made to interpret an unwritten source such as CIL the interpretative reasoning will inevitably need to refer back to the elements of the lawmaking process and as such be circular.Footnote ⁹ In a similar vein, it is posited that CIL rules do not require interpretation because the mere process of their identification delineates their content as well.Footnote ¹⁰

The argument that CIL is not subject to interpretation because it is unwritten is problematic. It is not entirely clear why the absence of a written textual manifestation in the context of CIL rules would imply that a CIL rule should not be subject to interpretation. An absence of a written manifestation merely means that a rule is not codified; it does not however deprive this rule of other forms of linguistic expression (e.g., oral expression) or of content, and subsequently of the need to clarify this content for the purpose of application in a given legal and factual context. Furthermore, in international law there is no universal approach which dictates that the unwritten character of a particular source precludes it from interpretation. For instance, as has been established by the International Law Commission (ILC) in its ‘Guiding Principles Applicable to Unilateral Declarations of State’,Footnote ¹¹ unilateral declarations, which may be formulated orally,Footnote ¹² are subject to interpretation if their content is unclear.Footnote ¹³ Similarly, with respect to general principles of international law, which are also themselves unwritten,Footnote ¹⁴ scholars seem to acknowledge, albeit in a more limited manner, that this source of law may be subject to interpretation.Footnote ¹⁵ Therefore, it cannot be concluded that the unwritten character of CIL automatically implies that this source of law is not subject to interpretation. Moreover, it is reasonable to observe that unwritten sources, as opposed to written ones, contain a higher degree of vagueness as a result of their unwritten character. This certainly seems to be the case with CIL, where scholars often point to this source’s inherent abstractness.Footnote ¹⁶ This would in turn imply that unwritten sources, rather than not being subject to interpretation, require precisely the exercise of interpretation in order to grasp their otherwise elusive content.

Turning to the second line of argument, it must be observed that this claim is negated by the practice of international courts which regularly engage in CIL interpretation as separate from its identification. For instance, in Mondev International Ltd v. United States of America, the Arbitral Tribunal observed that: ‘the question is not that of a failure to show opinio juris or to amass sufficient evidence demonstrating it. The question rather is: what is the content of customary international law providing for fair and equitable treatment and full protection and security in investment treaties?’Footnote ¹⁷ The tribunal then proceeded to interpret the customary rule of fair and equitable treatment.Footnote ¹⁸ Similarly, in its judgment in the Frontier Dispute case which dealt with the customary principle of uti possidetis, the International Court of Justice (ICJ), after briefly pointing to the ‘elements of uti possidetis’,Footnote ¹⁹ turned to an interpretation of the principle for the purposes of the case at hand.Footnote ²⁰ In addition to these examples which illustrate a clear distinction between identification and interpretation, courts more generally and regularly engage in the interpretation of CIL. Examples are replete from the dockets of the ICJ,Footnote ²¹ the International Tribunal for the Former Yugoslavia (ICTY),Footnote ²² and the World Trade Organization Appellate Body (WTO AB)Footnote ²³ to name a few.

Beyond the identification of examples where judges engage in the interpretation of CIL, accounting for the process of CIL interpretation bears a lot of theoretical relevance as well. In the absence of an interpretative process, there is no explanation about what happens to a CIL rule after it has been identified. Namely, once a rule of CIL is identified for the first time through an assessment of state practice and opinio juris, its existence is not restricted to the case where it was identified for the first time, but is rather a continuous one. When the same rule is invoked in subsequent cases before the same or a different judicial body, the judicial body does not usually go into the exercise of re-establishing that the rule in question is a customary one.Footnote ²⁴ Instead, the rule is interpreted within the given legal and factual context of the case at hand. In this sense, interpretation accounts for the CIL rule after its identification and before its application in a subsequent case. Arguing that CIL is not subject to interpretation thus fails to account for the continued existence and operation of a CIL rule after its first identification, and rather operates from the paradoxical premise that a rule of CIL should be identified each and every time anew.

This chapter accounts for the process of CIL interpretation through the illustrative tool of a ‘CIL timeline’ (Figure 21.1).Footnote ²⁵ The CIL timeline begins with the formation of a customary rule through the constitutive elements of state practice and opinio juris. The rule is then identified by an inductive analysis of these two elements. The reasoning in this stage includes the evaluation of evidence of practice and opinio juris and provides an answer to the question: does a customary rule exist? The outcome here is a binary one, in the sense that the answer will either be ‘yes, a customary rule exists’ or ‘no, a customary rule does not exist’. This reasoning however does not provide an answer to the question ‘is this customary rule applicable to the case at hand, and if yes, how is it applicable?’. This question is answered at the later stage of the CIL timeline, that is, at the stage of interpretation.

Figure 21.1 The CIL timeline

It is important to note that a form of interpretation may also be said to take place at the stage of identification. However, at this stage the relevant authority does not interpret a customary rule (as this rule has not been identified yet) but rather interprets the evidence of state practice and opinio juris in order to evaluate them and ascertain whether a rule has been formed. This distinction is particularly important, because although some authors have used the term ‘interpretation’Footnote ²⁶ for the reasoning that takes place at the stage of identification, this may not be considered as interpretation stricto sensu.Footnote ²⁷ This is because the exercise of weighing and measuring evidence of practice and opinio juris in order to discover whether they can be counted for the purpose of establishing a CIL rule is not the same as applying and interpreting the CIL rule to the legal and factual context of a case. The former is an exercise of evaluating evidence; the latter is an exercise of applying a formulated legal rule to a particular context of a case. In the former we ask questions such as: ‘does this piece of (state) behavior count as practice or opinio juris?’, ‘is this practice sufficiently widespread?’, ‘does this piece of evidence constitute a manifestation of opinio juris?’, ‘does this collection of practice and opinio juris point towards the existence of a rule?’, etc. In the latter we ask questions such as ‘is this CIL rule applicable to the factual context of the present case?’, ‘how does this CIL rule play out in the present context?’, ‘what is the specific content of this general CIL rule?’, etc. Thus, while the exercise of CIL identification may in fact also contain interpretative reasoning, this is not the same type of interpretation as the one exercised over an already identified CIL rule.

Even if one would concede that in the phase of interpretation the relevant interpreter may rely on some of the evidence of state practice or opinio juris from the phase of identification, this would still not constitute a counterargument to the overall claim that CIL rules are in fact interpretable.Footnote ²⁸ This is because, in this scenario, for the lack of a better analogy, this interpretative behaviour could be likened to how sometimes in the interpretation of a treaty interpreters may rely on the preparatory texts to elucidate intent, object and purpose, etc. Thus, an interpreter of a CIL rule might look back at evidence of state practice or opinio juris in the course of their interpretation of the rule, to answer some questions such as ‘what prompted the formation of this customary rule?’, ‘what is the aim to be achieved with this rule?’, or ‘can we discern specific sub-elements of this rule if we look back to past behavior?’.

Once a rule is identified by a relevant authority, every subsequent invocation of that rule in following cases is not an exercise of re-identification but rather of application and interpretation. The reasoning employed at the stage of interpretation is concerned with the determination of the content of the CIL rule and how this rule applies to the case at hand. Unlike the binary outcome of the identification stage, this reasoning may have a variety of outcomes depending on the rule being interpreted and the legal and factual circumstances it is being interpreted in. By distinguishing the two stages in this way, the CIL timeline illustrates the fact that the interpretation of a CIL rule is a process which manifests in a different and separate way from identification, a process which is subject to a separate (and perhaps unique) methodology, and a process which merits its own separate study (Figure 21.1).

In its 2016 Preliminary Report, the Study Group on Content and Evolution of the Rules of Interpretation flagged CIL interpretation as a relevant topic of exploration.Footnote ²⁹ Building on this recommendation, and observing the existing gap posed by the lack of guidelines for CIL interpretation, this chapter now turns to its central discussion on how domestic interpretive practice may be instructive to the development of rules or guidelines for CIL interpretation in international law.

3 The Role of Domestic Courts in the Development of International Law

For the purposes of this section, the role of domestic courts in the development of international law is examined along two broad lines of inquiry: the contribution of domestic courts to international law within the framework of sources (Section 3.1), and the informal contribution of domestic courts to international law beyond or outside the framework of sources (Section 3.2). The distinction of formal versus informal contribution employed in this section is used broadly and without prejudice to the more focused discussion on formalism and the sources of international law.Footnote ³⁰ The distinction is drawn with the aim of juxtaposing on the one hand the contribution of domestic courts to the development of international law primarily from within the framework of sources, and on the other hand the contribution of domestic courts to the development of international law beyond the framework of sources and in informal ways.

4 The Role of Domestic Courts in the Development of Rules for CIL Interpretation

Having examined the role of domestic courts in the development of international law both within the framework of source and beyond it, we now turn to the operative portion of this chapter. What the above examination demonstrates is that there is ample scholarship to draw from when examining the relationship between domestic courts and international law. However, as the reader might have already noticed, the majority of scholarship focuses on the potential contribution of domestic courts in the form of substantive legal analysis and content. Conversely, what seems to be lacking is an account of the ways in which the interpretive methodologies of domestic courts may contribute to the development of interpretive methodologies in international law. In this section, we will first examine several cases where domestic courts engaged with CIL or domestic custom, with a particular focus on the methods of interpretation they employed, and, where available, the rationale behind that methodological choice (Section 4.1). Then, the section will lay out a set of preliminary observations on how these examples by domestic courts may contribute to the development of rules for CIL interpretation in international law along two lines of inquiry: how can we learn from domestic interpretive practices? (Section 4.2) and why should we learn from them? (Section 4.3).

5 Concluding Remarks

Domestic courts have the potential to contribute significantly in the development of rules or guidelines for the interpretation of CIL. Scholars have demonstrated that domestic courts are in fact often faced with the task to apply and interpret CIL, and thus yield relevant practice from which we may learn in the study of CIL interpretation. Moreover, a brief survey of some domestic practice indicates that domestic courts employ a variety of methods to interpret customary law, and there is a lot to learn from and examine in these methodological choices.

This chapter began with the question: how can interpretive methodologies employed by domestic courts inform the development of rules or guidelines for the interpretation of CIL? It examined the general scholarship on the role of domestic courts in the development of international law, and applied these findings particularly to the potential contribution of interpretive methodologies of domestic courts to the development of interpretive methodologies in international law. By examining five cases from various jurisdictions the chapter observed that in the interpretation of custom domestic courts may employ teleological, systemic or evolutive interpretation. While this is a restricted sample and no grand conclusions may be reached yet, these cases open many interesting questions about the nature of customary law interpretation and the role of interpretation in the genesis and continued existence of customary rules.

Bearing this in mind, the chapter laid out some preliminary observations along two lines of inquiry: (i) how can we learn from domestic interpretive practices? and (ii) why should we learn from them? With respect to the first question, it was observed that in order to adequately study the contribution of domestic courts’ practice to the identification and development of rules for the interpretation of CIL we should depart from the role of domestic courts within the sources framework and build a broader framework of analysis from there. Thus, additional informal normative and conceptual points of interaction need to be considered in order to register and account for all the ways in which domestic interpretive practices can inform our inquiry. In answering the second question, the chapter submitted three reasons why we should look to domestic practice. Firstly, because the interpretation of CIL is currently an under-examined and unregulated sphere of international law, and this kind of ‘gap’ legitimately invites contributions from domestic practice. Secondly, because domestic courts are frequently engaging with customary law, and this provides a growing pool of relevant interpretive practice which can be instructive to the development of rules for CIL interpretation in international law. Finally, because by learning from domestic practices for the purpose of CIL interpretation, international law can provide domestic judges with various familiar tools for their further engagement with CIL in the domestic context. This is beneficial both for domestic judicial practice and for the further development of international law.

Overall, the chapter found that this is an area of research which raises various relevant questions, and thus invites substantive further investigation.

1 Introduction

The TRICI-Law project observes that ‘in the study of customary international law (CIL) there is a critical gap in understanding how CIL can be applied in individual cases once it has been formed’. The project then sets for itself the goal to uncover rules of interpretation of CIL. In the words of the project, if such rules were to exist, CIL need not be induced (ascertained) each and every time, by reference to state practice and opinio juris or asserted by judges.

This chapter attempts to narrow the gap in understanding how CIL is applied and interpreted by domestic courts. Domestic courts are important agents of international legal development,Footnote ¹ and they contribute to the entrenchment of the rule of international law, including CIL.Footnote ² Accordingly, a study of the interpretation of CIL cannot do without an analysis of domestic court practices.

The contribution opens with a critical reflection on the proposed doctrinal shift from mere CIL ascertainment to interpretation of more or less stabilised CIL norms (Section 2). As domestic courts tend to apply pre-existing CIL rather than ascertain CIL de novo,Footnote ³ the author sees a window of opportunity for CIL interpretation. He then goes on to ascertain whether domestic courts also use this window in practice. He does so by analysing a large data set of domestic court decisions (Section 3). The empirical analysis yields a number of ‘true positives’ which suggest that, in admittedly rare cases, domestic courts genuinely interpret relatively stable, pre-existing CIL norms, in particular in the area of international immunities. These courts appear to use methods of interpretation that reflect those used for treaty interpretation, notably systemic interpretation and interpretation taking into account subsequent practice.

2 From CIL Ascertainment to Interpretation

The quest for rules governing the interpretation of norms of international law other than treaty-based norms is not new. Reference can notably be made to the interpretation of the text of resolutions of the United Nations Security Council (UNSC).Footnote ⁴ The Advocate General advising the Dutch Supreme Court, for instance, recently opined that ‘while Article 31 VCLT did strictly speaking not apply to a resolution of the UN Security Council, its rule of interpretation can be considered as a rule of customary international law’.Footnote ⁵ The reasoning appears to be that, precisely because Article 31 of the Vienna Convention on the Law of Treaties (VCLT) is of a customary nature, it can also be applied to the interpretation of sources of international law other than treaty law, such as UNSC resolutions. If that is the case, nothing stands in the way of applying the rules of interpretation laid down in Article 31 VCLT to CIL as well.

This line of argumentation is not necessarily convincing, however. There may be little doubt regarding the customary character of Article 31 VCLT,Footnote ⁶ but that does not make the rules of interpretation laid down in that provision applicable to sources of international law other than the treaties which the VCLT is supposed to govern.Footnote ⁷ In fact, that the relevant rules of Article 31 VCLT are customary means, in the first place, that they can be applied to other treaties that are not governed by the VCLT, for example, because they predate the entry into force of the VCLT in 1980, because the state party to the relevant treaty has not ratified the VCLT, or because the treaty does not fall within the scope of the VCLT (for instance because it has been concluded in oral form, or between states and other subjects of international law, or between such other subjects inter se).Footnote ⁸ After all, Article 31(1) VCLT specifically stipulates that ‘[a] treaty shall be interpreted in good faith’.Footnote ⁹ If that rule has customary character, the parallel customary rule should also state ‘a treaty shall be interpreted in good faith’.

Nevertheless, this does not mean that Article 31 VCLT has no relevance for the interpretation of norms from other sources of international law. It may have such relevance, as a material source of inspiration, or via reasoning by analogy. In all likelihood, the VCLT rules of interpretation should not be transposed lock-stock-and-barrel to the interpretation of norms derived from other sources of international law, to paraphrase Arnold McNair’s warning in the International Court of Justice (ICJ) South West Africa advisory opinion not to simply import domestic law institutions into international law.Footnote ¹⁰ Rather, when considering transposition, one may have to bear in mind the special features of other sources of international law compared to treaty law. Thus, Sir Michael Wood has sympathy for interpreters’ reliance on Article 31 VCLT when interpreting UNSC resolutions, but, given the more political nature of this source of law, invites the interpreter to pay specific attention to the circumstances in which the resolution has been adopted as well as the context of the UN Charter.Footnote ¹¹

That the rules of interpretation devised for treaties can apply mutatis mutandis to UNSC resolutions is in any event understandable to the extent that a binding UNSC resolution is, just like a treaty, a written source of international law. Moreover, UNSC resolutions find their legal basis in a treaty (the UN Charter).Footnote ¹² It is less self-evident to apply Article 31 VCLT, with the necessary modifications or not, to the interpretation of CIL norms. Unlike a treaty or a UNSC resolution, CIL is an unwritten source of international law, and it does not, at least not formally, find its legal basis in a treaty. The material source of CIL may sometimes be a treaty, for example, because subsequent to the adoption of a treaty norm, state practice and opinio juris converge on the content of that norm, but at the end of the day, for its legal existence the customary norm is not dependent on the treaty norm.Footnote ¹³ Because CIL is an unwritten, flexible and protean source of international law, it does not easily lend itself to the transposition of rules of treaty interpretation. What is more, the question may arise whether rules of interpretation of customary law norms serve any purpose at all, as CIL is – at least potentially – in a state of constant flux. Interpretation of norms only makes sense if those norms have a stable existence. In the classic understanding of ascertainment and identification of norms of CIL, legal authorities (law-applying or law-ascertainment agencies) always have to revisit the very existence of customary norms de novo. Although unlikely, it is after all not impossible that customary norms change or form almost overnight (instant custom).Footnote ¹⁴

This also appears to follow from the very text of Article 38(1)(b) of the ICJ Statute, which provides that the ICJ (and courts more generally one may well posit) ‘shall apply’ … ‘international custom, as evidence of a general practice accepted as law’. Pursuant to this provision, courts apply a customary norm as soon as they have established its evidence-based existence, without any need for interpretation stricto sensu. This process may perhaps appear interpretative, in that judges interpret evidentiary materials placed before them with the aim of distilling customary norms from those materials. But such interpretation takes place only in an evidentiary rather than normative sense. Judges do not interpret previously crystallised norms by analogy with Article 31 VCLT; they simply ascertain the law. Thus, Merkouris observes that judges do ‘not interpret State practice, they evaluate it, they examine its gravity for the purpose of determining the existence or not of CIL’, whereas ‘interpretation of CIL requires an already existing CIL rule’.Footnote ¹⁵

This process of CIL ascertainment or identification has been the subject of many studies, most recently by the International Law Commission (ILC).Footnote ¹⁶ In a previous publication with a co-author, this author systematised and categorised the variegated CIL ascertainment techniques used by domestic courts.Footnote ¹⁷ Triggered by Stefan Talmon’s earlier finding that, ‘when determining the rules of customary international law, the ICJ does not use one single methodology but, instead, uses a mixture of induction, deduction and assertion’,Footnote ¹⁸ it was examined whether similar processes could be witnessed in domestic courts. An analysis of a large number of recent domestic court cases bore out that this is indeed the case. Domestic courts do not normally identify CIL norms on the basis of the textbook method of ascertaining a general practice accepted as law. Rather, they tend to outsource the determination of custom to treaties, non-binding documents, doctrine or international judicial practice. Sometimes, these courts simply assert, without citing persuasive practice, the existence of a customary norm.

While, in principle, ‘other authorities’ only have evidentiary value that should be weighted with other materials which more inductively evidence (or not) the existence of a particular customary norm, one cannot escape the impression that domestic courts are simply giving effect to, or applying pre-existing customary norms, that is, norms which have been identified earlier. But if that is true, there is in principle room for the development of rules of interpretation. As Merkouris observed: ‘[O]nce CIL has been identified as having been formed, its continued manifestation and application in a particular case will be dependent on the deductive process of interpretation. In this manner, interpretation focuses on how the rule is to be understood and applied after the rule has come into existence and for its duration.’Footnote ¹⁹ If domestic courts are in fact interpreting customary norms when applying them in given cases, our earlier publication’s lament that domestic courts failed to engage in a serious CIL ascertainment process (which includes parsing all available materials),Footnote ²⁰ loses some of its force. Indeed, assuming that customary norms existentially stabilise at one point, after which they are simply interpreted, there is no need for an elaborate process of identifying a customary norm de novo. Instead, courts may satisfy themselves with reaffirming the existence of the norm – presumably established by other law-ascertainment agencies at an earlier stage without subsequently being challenged – and instead concentrate on how to interpret the norm in a manner similar to how treaty interpretation takes place. Specific CIL rules of interpretation that are autonomous from the VCLT rules of interpretation can, in principle, develop via the regular customary process, through concurrent state practice and opinio juris.Footnote ²¹ Merkouris has argued that such rules already exist, and that they themselves are amenable to interpretation.Footnote ²²

3 The Practice of Domestic Courts Interpreting CIL

The author’s earlier research on how domestic courts found and applied customary norms was conducted through the lens of ascertainment. The current contribution, revisits relevant court decisions with a view to understanding more in-depth how domestic courts engage in CIL interpretation.

Oxford University Press’s database International Law in Domestic Courts (ILDC) was used as the main resource to find relevant domestic court decisions. ‘Interpretation’ was used as the search term, combined with the generic subject ‘Sources, foundations and principles of international law’. The headnote of the search results subsequently indicates whether CIL was relevant to the domestic court decision.Footnote ²³ Also, ILDC marks the search term – in this case ‘interpretation’ – in the summary and text of the decision, which greatly facilitated the research.Footnote ²⁴ Methodologically, a discourse analysis of written texts (judgments) was carried out;Footnote ²⁵ the research analysed to what extent domestic courts explicitly used the term ‘interpretation’ when applying CIL.Footnote ²⁶ Such an analysis has its limitations in that it may discount practices of courts implicitly interpreting customary norms. Accordingly, it also included references to interpretation by the ILDC commentators directly commenting on the judgments. However, the emphasis does not lie on what courts may have meant when applying customary norms, but primarily on what they did in fact: did they consciously consider customary norms to be amenable to interpretation?

The search yielded a number of domestic court decisions which featured both ‘customary international law’ and ‘interpretation’. However, not all of these results pertain to the interpretation of CIL norms proper. Such results are ‘false positives’.Footnote ²⁷ A first category of false positives comprises those decisions in which domestic courts erroneously use the term ‘interpretation’, when they in fact meant something else, in particular ascertainment. A second category of false positive comprises those decisions in which courts do engage in interpretation, but not of CIL, but rather of domestic (statutory) law, although in light of CIL. These two categories of false positives are briefly discussed in Section 3.1. Subsequently, Section 3.2 proceeds to the core analysis of true positives, that is, decisions in which courts genuinely interpret CIL norms.

4 Concluding Observations

By and large, domestic courts, just like international courts, hew to the fiction that they find, identify or ascertain CIL. Earlier research has demonstrated that domestic courts have only limited agency in identifying CIL.Footnote ⁸⁸ Instead, they tend to simply apply pre-existing CIL. However, when domestic courts apply such CIL, they may also interpret and develop CIL, as any application of law, almost out of necessity, also involves a measure of interpretation and legal development.Footnote ⁸⁹ This contribution supports the TRICI project’s methodological premise that CIL norms, just like treaty norms, can be interpreted. Interpretation will notably take place after a ‘core’ CIL norm has crystallised and stabilised, after which the penumbra of that rule – its precise scope, its exceptions – are amenable to mechanisms of interpretation.

This chapter has analysed a large dataset of domestic court decisions relevant to CIL, and found that, indeed, domestic courts at times engage in CIL interpretation, even if they largely refrain from using that term. Domestic courts may interpret CIL autonomously, may defer to and validate international courts’ CIL interpretations, or they interpret written documents, such as treaties, codifying CIL norms. Such practices bear out that domestic courts may consider some core CIL norms to be relatively stable and amenable to further refinement through interpretation.

When interpreting CIL, domestic courts appear to resort mainly to systemic interpretation and interpretation on the basis of subsequent state practice. This reflects earlier findings by Panos Merkouris with respect to CIL interpretation by international courts.Footnote ⁹⁰ In particular, domestic courts apply by analogy the canons of construction laid down in Article 31(3)(b) VCLT (‘any subsequent practice in the application of the treaty which establishes the agreement of the parties regarding its interpretation’), and Article 31(3)(c) (‘any relevant rules of international law applicable in the relations between the parties’). The analysis of state practice under the CIL equivalent of Article 31(3)(b) VCLT tends to be cursory, however. As Odile Ammann has pointed out, mainly in respect of CIL ascertainment, domestic courts tend to refer to their own practice or the practice of their own state, a process which she characterises as self-referentiality and circularity.Footnote ⁹¹ Cast in terms of interpretation, such a process may take the form of ‘reverse consistent interpretation’: domestic courts may choose this interpretation of a CIL norm which is mostly in line with domestic law and practice.Footnote ⁹²

In future cases, domestic courts deciding cases on the basis of CIL may in any event want to be more explicit regarding whether they engage in de novo CIL norm-identification or rather in the interpretation of pre-existing and stabilised customary norms. In case of interpretation, they may want to improve the methodological quality of their reasoning by pinpointing the canon of construction which they apply (e.g., systemic interpretation; interpretation on the basis of subsequent practice). Finally, in the case of both CIL identification and interpretation, they should make sure that they rely on sufficient international (including foreign) legal practice, in keeping with the methodological requirements of CIL identification and interpretation.

1 Introduction

Although the relevance of treaty law has grown over time in international law, customary rules remain an important source of law in international relations. An underdeveloped theme when studying customary norms is the use of customary international law (CIL) within domestic legal orders. Especially in federal states, where the composing parts of the polity are sometimes seen as ‘remnants’ of or reminiscent to sovereign states, rules pertaining to the relationship between states within the field of international law might also prove useful for regulating the behaviour of states that share at least some of the characteristics of sovereign entities but have entered into compacts merging them into greater and tighter-knit polities. For a constitutional court in a federal state, such norms therefore might be a useful tool in solving disputes between the states or between the federation and the states. They might also be useful in regulating treaty practices in federations where there exists a possibility of interstate treaties. Yet, this ambiguity (is the legal order that unites the states and the federation through the federal constitution all-encompassing, or does it leave room for the use of non-domestic norms?) concerning the relationship between partners in a federal makeup can also be shown in the way in which domestic courts make use of these norms. It is seldom completely clear-cut whether a court actually does use a rule of CIL in its original meaning.

In this chapter, I will try to shed some light on the way in which rules of CIL have been used within the domestic legal order for regulating the relations between the states of both federal entities in Germany and Austria. Other federal states might have been used instead of these two countries. The reason for choosing Germany and Austria is that in the former an extensive debate has been held on the question whether the German states that entered into the federal compact of 1871 could still be described as states despite the fact that they had lost their independent status under international law.Footnote ¹ This debate created an intellectual and legal atmosphere in which the statehood of the German regional entities remained relevant, even after the end of the German empire in 1918 – and thus the idea that the rules pertaining to the relationship between states, that is, international law, could still be relevant as well. Austria only became a federal state in 1920, after the demise of the Habsburg empire and thus more or less followed the ideas and conceptions already developed in Germany. Moreover, in both Germany and Austria, domestic interstate law is primarily created through treaties, which also points in the direction of an interesting analogy to international law.

As will be demonstrated, rules of CIL have (arguably) shown to be useful, especially when it comes to disputes between two or more states. As stated above, the use of these norms does raise important questions concerning the nature of federal legal orders, however. Are the component parts of a federal state still ‘states’ in an international law sense of the word? If so, how much legal manoeuvring space exists to make use of norms of international law to regulate their relations? What is the legal justification, if any, for the ‘domestic’ use of norms originating under international law? These questions, and the ambiguous unease they provoke in domestic courts, will be dealt with in the following paragraphs.

In the next section, the focus will be laid on the way in which in the Weimar Republic rules of CIL were used to solve disputes between German states. This will be done through an analysis of the case law of the special court constituted under the 1919 German constitution to deal with such legal questions, the Staatsgerichtshof für das deutsche Reich (RStGH). In the third section, the focus will be on the post-war case law of the Federal Constitutional Court, the Bundesverfassungsgericht (BVerfG). The question will be analysed how the case law of the BVerfG has built upon the foundations laid in the 1920s by its Weimar predecessor. The fourth section deals with the situation in Austria: interestingly enough, the Austrian constitution (the Bundes-Verfassungsgesetz, B-VG) contains an article explicitly acknowledging the existence and the relevance of international law within the domestic legal order to regulate interstate and federal-state relations, Article 15a B-VG. This article and its practical use under the Austrian constitution will be scrutinised. Finally, some conclusions will be drawn and some light will be shed on the question whether or not courts are using norms of CIL, or are merely using them as a source of inspiration to develop unwritten principles of federal law.

2 The Use of CIL in the Republic of Weimar

After the fall of the monarchy in Germany in November 1918, the new republican government organised elections for a federal constituent assembly, which assembled for the first time on 6 February 1919 in the Thuringian city of Weimar.Footnote ² Over the following months, it elaborated a new constitution for the German Reich that introduced general suffrage for men and women, a catalogue of fundamental rights, a full parliamentary system and a strong, directly elected federal president with far-reaching powers. The constitution was enacted on 11 August 1919. The Reich constitution (often described as the ‘Weimar’ constitution because it was created there)Footnote ³ maintained the existing makeup of the German state as a federation, but it contained many characteristics that strengthened the role of the central authorities. Notable among those were the fact that the new Reich government would be dependent on the confidence of the majority of the directly elected Federal Diet,Footnote ⁴ representing the whole of the German nation and that the new Reich president was also elected by the whole nation,Footnote ⁵ but it was also visible in the division of powers between the federation and the states: compared to Bismarck’s 1871 constitution, the legislative and executive powers of the federation were visibly strengthened.

Among the ‘federal’ innovations of the Weimar constitution we also find the introduction of a new federal court, the so-called Staatsgerichtshof für das deutsche Reich (Federal State Court, RStGH) a specialised court to deal with disputes of a federal nature. Article 19 of the constitution described its legal powers. The RStGH was empowered to settle disputes of a public law nature within one of the German states, between two or more states or between the Reich and one or more states, insofar as no other court was created to deal with any of these issues.Footnote ⁶ It was originally envisioned as being an adjacent court to the proposed Federal Administrative Court, the Reichsverwaltungsgericht. This, however, was never actually created, which led to the situation that the court became permanently organised along the lines originally meant as a temporal solution: it was an ad hoc court, annexed to the Reichsgericht. Its president was the president of the Reichsgericht itself, and it consisted of six other members: three judges of the Reichsgericht, one judge from the Oberverwaltungsgericht of the state of Prussia, one judge from the Verwaltungsgerichtshof of the state of Bavaria and one judge from the Oberverwaltungsgericht of the state of Saxony.Footnote ⁷ Thus, the RStGH was a mixture of ‘ordinary’ and administrative judges.

Although case law concerning constitutional disputes within a state formed the bulk of the activities of the RStGH (the majority of the German states, including the largest and most populous of them all, Prussia, did not create their own constitutional courts), it did decide a number of interesting cases concerning disputes between different states. And it was especially in this field that the court used norms of CIL to settle these disputes, insofar as the domestic legal order did not provide sufficiently clear or relevant norms. The basis of the use of customary norms was found in Article 4 RV. This article stated: ‘Die allgemein anerkannten Regeln des Völkerrechts gelten als bindende Bestandteile des deutschen Reichsrechts.’Footnote ⁸ This article was a novelty in German constitutional law: under the old constitution of 1871, international law was solely seen as the law between states: for the citizens of Germany, it formed a res inter alios acta. That changed because of Article 4 of the new 1919 constitution: for the first time, norms of international law became part of the domestic legal order, binding public bodies and citizens alike and gaining relevance before the German courts.Footnote ⁹ It was never entirely clear which norms of international law were covered by Article 4: only norms of CIL, or also treaty norms? Those acknowledging that treaty law could be covered by Article 4 RV mostly accepted that the criterion for ‘acknowledgement’ by Germany entailed the ratification of the treaty through a federal act, as provided for by Article 45 (3) RV.Footnote ¹⁰ Those denying it mostly adhered to the idea that Article 45 (3) RV itself regulated the transformation of treaty norms into the German legal order. Since according to both theories Article 4 RV regulated the transformation of customary norms and according to both theories the internal hierarchical status of all international norms was that of a federal act, the question did not have huge practical relevance.Footnote ¹¹ Being on a par with federal acts, customary norms of international law took precedence over earlier federal acts on the basis of the lex posterior rule and took precedence over all state law (including constitutional state law) on the basis of the lex superior rule.Footnote ¹²

The RStGH never chose sides in the debate on the specific relationship between Article 4 and Article 45 (3) RV. In four of the cases it decided on disputes between two or more of Germany’s states it made use of Article 4 RV to settle the case in a legally binding matter. The first of those was a dispute between Prussia and Bremen concerning a 1904 treaty between the two states on the legal and economic status of Bremerhaven. Bremen claimed before the RStGH that this treaty had a very negative impact on the Bremen economy because it was heavily written in favour of Prussia’s interests in the region. Under the circumstances of the old Reich this could perhaps be justified, Bremen claimed, because under the old constitution Prussia had been more than a primus inter pares.Footnote ¹³ But under the new 1919 constitution this had changed and Prussia had become nothing more than just the largest of the German states. Thus, the 1904 treaty should not be used under the same circumstances as before the war, Bremen argued before the court. The second one decided by the RStGH concerned a dispute between Baden on the one hand and Württemberg and Prussia (for its Hohenzollern territories bordering Baden and Württemberg) on the other concerning the use of water rights from the river Danube. The third one was a case concerning Bremen on the one hand and Prussia, Brunswick and Thuringia on the other hand concerning the pollution of the river Weser by potassium mines in the latter states, forcing Bremen to halt the use of the river as a source of drinking water. The fourth and final case in which the RStGH used Article 4 RV was a dispute between the states of Lübeck and Mecklenburg-Strelitz on fishing rights in the Lübecker Bucht, a part of the (German) Baltic Sea.Footnote ¹⁴

The second decision mentioned above is a good example of the way in which the RStGH made use of Article 4 of the 1919 constitution in all four of these cases and is therefore worthwhile looking into in some more detail. The case centred around the fact that water from the Danube leaked away on the territory of Baden, ending up in the small Badener river Aach. This river was an important source of drinking water in parts of Baden, and the Badener authorities had a keen interest in the continuation of this leakage. This, however, led to measurable lower levels in the Danube during periods of lesser rainfall. Both Württemberg and the Hohenzollern province of Prussia used the Danube for drinking water and industrial purposes too. They claimed that Baden strengthened the natural seepage through lack of confining measures on the banks of the river and even through active measures, thereby infringing on the rights of both states. They brought a case before the RStGH in order to compel Baden to desist from any further measure that would strengthen the natural seepage of the Danube waters. The RStGH gave its decision on 18 June 1927.Footnote ¹⁵ The court concludes in a preliminary remark that the conflict is not of a private law, but of a public law nature, since the conflict deals with the way in which the three states conduct their water policies: and those powers were not conferred to the federation by the Reich constitution, Thus, the conflict was of an interstate, public law nature, making it one of the types of dispute for which the Staatsgerichtshof enjoyed competence under Article 19 RV. The court further concludes that neither federal law nor Badener law can be used as a legal basis for settling the claim, an analysis that leads to one of the key paragraphs of the decision.

When neither federal nor state law is available, the court must make use of other sources of law to settle the dispute, since there can be no gap in the legal order (Rechtslücke), the court states.Footnote ¹⁶ Only international law can play that part. It can be used in the interstate relations within Germany, albeit in a limited way. For although the relationship between the German states is primarily regulated and structured through the federal constitution, the Reich never intended to create an all-encompassing order. Fundamentally, the German states are still (sovereign) states, albeit with a limited sovereignty. Article 5 RV states that the state powers are enacted through state organs on the basis of the state constitutions. Articles 6 frth. RV regulate the legislative powers of the federation in such a way that the states enjoy all legislative competences unless and insofar the federal legislator has been made competent. Insofar as the states have retained legislative competences, they also have the right to enter into treaties with foreign powers (Article 78 (2) RV). Thus, there is a clear basis, through the whole structure of the federal makeup of the 1919 constitution and through Article 4 RV itself, to make international law applicable on the interstate relations of the German states.Footnote ¹⁷

From this, the court draws some important conclusions. It claims that international law, in its more recent developments, has strengthened the idea that states are limited in their sovereignty because they belong to an international order of states. It follows that there exists a general (unwritten) principle of mutual respect and good neighbourliness and a duty of non-harm. Within the German legal order, this is even more the case: the preamble of the 1919 constitution states that it originates from the unity of the German nation itself, with the aim to promote the inner peace of Germany. The constitution also creates through Article 110 (2) the right to equal treatment in every German state for all German citizens. From this it follows, the RStGH argues, that every German state has a duty to act in such a manner as not to infringe upon the rights and interests of Germans in other states unless this is absolutely necessary. The sovereignty of the German states on their own territory is therefore even more limited than the limitations that would follow from the aforementioned general principle of international law, because the German states form a legal community that is more close-knit than an ordinary international one.Footnote ¹⁸

The use of CIL by the RStGH is therefore of a somewhat ambiguous nature. On the one hand, the court fully recognises that Article 4 of the federal constitution regulates the domestic force of (customary) international norms. It acknowledges that the principle of friendly co-existence and non-harm to other states is a norm that is valid, applicable and enforceable within the domestic legal order of Germany. On the other hand, however, the court clearly accepts the idea that because of the fact that the relationship between the German states is not primarily of an international legal character, it is primarily domestic, federal law that regulates interstate relations. Furthermore, customary international norms have a different, and more material content than they would have in the international legal order. One could perhaps argue (although the Staatsgerichtshof is silent on the matter) that Article 4 RV not only transforms norms of CIL into German law but also – dependent on the norm – adapts their content to make them better suited for domestic use.

3 The Use of CIL in the Federal Republic of Germany

This ambiguity has not been clearly solved in the post-war legal order of the Grundgesetz either. One of the very first decisions the Bundesverfassungsgericht gave after its inception was the so-called Südweststaatentscheidung of 23 October 1951.Footnote ¹⁹ Although the new federal constitutional court had far more powers than the ones given to the RStGH, public law disputes between the federation and one or more states and between two or more states are enumerated among them.Footnote ²⁰ In that sense, the RStGH is a clear legal predecessor of the BVerfG. The Südweststaatentscheidung originated in an agreement between France and the United States of July 1945 to demarcate their respective zones of occupation in the southwest of Germany along the federal highway (Reichsautobahn) Karlsruhe-Ulm-Stuttgart. This divided the existing territories of the old states of Württemberg and Baden (and the Prussian province of Hohenzollern-Sigmaringen) between the French and American zones of responsibility. The American military authorities created in their zone of occupation the new state of Württemberg-Baden; the French created two new states, Baden and Württemberg-Hohenzollern.

These three states enacted their respective state constitutions on 28 November 1946,Footnote ²¹ 18 May 1947Footnote ²² and 22 May 1947.Footnote ²³ All three states became part of the Federal Republic of Germany; their State Diets enacted the Basic Law on 18 and 21 May 1949 respectively. Almost immediately after the creation of the three states in the southwest it became clear that they were not only artificial creations, which bore no resemblance to the old territorial divisions in the region, but that they were all three too small to function properly in the new makeup of the Federal Republic of Germany. Especially Württemberg-Hohenzollern was a reminder of the Kleinstaaterei that was already partially abolished by the 1919 constitution (which had enabled the federal lawgiver to act on these matters in Article 18 RV, resulting in the creation of the state of Thuringia), but had for the rest completely disappeared under the post-1945 allied occupation.Footnote ²⁴ Despite pressure from the allied authorities and negotiations in the summer of 1948, no solution had yet been found when the Basic Law entered into force on 23 May 1949. The Basic Law did, however, contain a special provision for the reshaping of the southwest in Article 118. This article created a more expedient regulation for the possible merger of the three south-western states compared to the ‘normal’ procedure for the creation or dissolution of a state of Article 29 GG. In all three states a referendum was held (on 24 September 1950) about the question whether or not they should merge into a new Südweststaat. In Württemberg and in the old Hohenzollern lands a vast majority wanted the merger: but in Baden, a clear majority wanted the old state of Baden back, while in the old Badener parts of Württemberg-Baden a significant part of the voters (over 40 per cent) wanted this as well.

Since the three states could not come to an agreement, the federal legislator decided to act unilaterally through Article 118 of the Basic Law. One of the two federal acts published jointly on 4 May 1951 was an act that prolonged the tenure of the existing state diets of Baden and Württemberg-Hohenzollern until the day the state constitutions would be abolished because of the creation of the new state of Baden-Württemberg. This was deemed necessary because their tenure would end before this date and new elections for state diets that would only exist for a couple of weeks were seen as useless and onerous.

Both Baden and Württemberg-Hohenzollern had come to the same conclusion, however. They both started a procedure to amend their state constitutions in order to extend the tenure of their respective diets. Both state constitutions made a plebiscite compulsory for amending the state constitution; both plebiscites were planned for 8 May 1951. Thus, it could be argued, the federal legislator had regulated a matter that fell within the constitutional autonomy of the states. The government of Baden decided to bring the matter before the newly created Bundesverfassungsgericht. Baden not only claimed that this federal act violated the constitutional autonomy of the state, but it also claimed that the other federal act of 4 May 1951 (which regulated the merger of Baden, Württemberg-Hohenzollern and Württemberg-Baden into Baden-Württemberg) was unconstitutional as well, because the Badeners had clearly voted through the referendum in September 1950 that they wanted their old Baden back, not a new Baden-Württemberg. The federal constitutional court sided with the state government of Baden insofar as the federal act prolonging the tenure of the state diets was concerned. It concluded that since the Basic Law was founded upon the principle of democracy, the federal legislator could not interfere unilaterally with the tenure of a state diet in bypassing the constitution that the people of that state have given themselves democratically.Footnote ²⁵

With regard to the second act, the one creating Baden-Württemberg, the situation was different. Article 118 (2) of the Basic Law regulated that the creation of the new Südweststaat was only possible under the guarantee of a mandate of the people. The federal legislator therefore regulated that a referendum was to be held that asked two questions: do you want the new Baden-Württemberg? Or do you want the existing state of Baden? (or in Württemberg-Baden and Württemberg-Hohenzollern: do you want a state of Württemberg, including Württemberg-Hohenzollern?). Baden claimed that this infringed upon the rights to self-determination of the people of Baden because it was not possible for the Badeners to recreate the old pre-1945 state of Baden, although the majority of the Badeners had chosen that option in the 1950 referendum. The Baden state government defended the existence of this right to self-determination partially on the basis of Article 25 of the Basic Law, the successor to the old Article 4 RV. The main difference between the two articles is that Article 25 GG not only states that the general norms of international law are part of the federal legal order, but it also states that these norms create rights and obligations for the inhabitants of the federal republic and that they take precedence over federal acts and state law.Footnote ²⁶ Baden claimed that because of Article 25 GG, the general norm of the right to self-determination took precedence over said federal act. The BVerfG denied this claim: it stated, following the reasoning of the RStGH in the Danube decision of 1927, that (customary) rules of international law could only play a part in interstate relations, not in the constitutional relation between the federation and the states.Footnote ²⁷

The question which role – if any – CIL as transformed by Article 25 of the Basic Law did play in the interstate relations within Germany was decided by the federal constitutional court in a later decision, the Coburg decision.Footnote ²⁸ The case arose from a dispute between the cities of Coburg and Neustadt bei Coburg and the State of Bavaria. Saxony-Coburg, as has been stated above, was one of the Thuringian states. But while all the others had merged into the state of Thuringia by the Federal Act of 1 May 1920, Coburg (including the smaller city of Neustadt bei Coburg) had been merged with Bavaria through a state treaty (Staatsvertrag) between the two states of 24 February 1920.Footnote ²⁹ One of the clauses of this treaty between the states of Bavaria and Saxony-Coburg had been that Neustadt bei Coburg would be and remain a separate municipality under its own jurisdiction within Bavaria. Through the state decree on the new territorial division of Bavaria of 27 December 1971,Footnote ³⁰ Neustadt bei Coburg had been made a part of the district (Kreis) of Coburg. Both the cities of Coburg and Neustadt bei Coburg claimed that this decree violated the clause of the 1920 treaty that guaranteed Neustadt bei Coburg the right to remain its own district (kreisfreier Stadt) and in doing so also violated the 1946 Bavarian constitution, which stated in Article 182 that earlier state treaties to which Bavaria was a party would remain in force. As legal successors of the state of Saxony-Coburg, both municipalities invoked the right before the Bundesverfassungsgericht to represent the interests of the erstwhile state, which could no longer act and speak for itself. Coburg and Neustadt bei Coburg claimed that Bavaria acted in bad faith in invoking the customary rule of clausula rebus sic stantibus to terminate the 1920 state treaty: they stated that the circumstances since 1920 had not changed so much that it would entitle Bavaria to unilaterally abolish the guarantee given to Neustadt bei Coburg.

The court rejects the claim of Coburg and Neustadt bei Coburg. The BVerfG acknowledges the right of the two municipalities to speak on behalf of the no longer existing Free State of Saxony-Coburg.Footnote ³¹ It also rules that the state treaty of 1920 was still valid; the federal constitutional court underlines that the 1920 state treaty guarantees the city of Neustadt bei Coburg an autonomous existence without subordination to a district authority.Footnote ³² But, the Bundesverfassungsgericht argues, the clausula rebus sic stantibus does come into play, because the circumstances in Germany and Bavaria in 1971 differ vastly from those in 1920. It is, the court claims, as a rule regulating the behaviour of German states towards each other, an unwritten part of the German federal constitutional law. It is therefore not a rule of CIL transformed by Article 25 of the Basic Law. The BVerfG acknowledges that under the 1919 constitution the RStGH had ruled that norms of CIL could play a part in interstate relations, but under the Basic Law there is no longer any room for such rules: all norms regulating the federal makeup of Germany are German, domestic rules of constitutional law.Footnote ³³ Bavaria is therefore entitled to unilaterally change the status of Neustadt bei Coburg, despite the 1920 treaty.

So far, so good: in the post-war makeup of Germany there is no longer any place for rules of CIL to solve disputes between German states, although materially speaking the unwritten rules of German constitutional law that oblige the state organs to Bundestreue towards one another are basically the same as the rules of CIL invoked by the RStGH in the 1920s. Or is there? For the categorical denial of the relevance of rules of customary law in the Coburg decision did not hold for long. In its Grundlagenvertrag decision of 31 July 1973Footnote ³⁴ the BVerfG cast serious doubt on the steadfastness of its recent convictions. The constitutional court had to decide on the constitutionality of the Federal Act ratifying the treaty between the Federal Republic of Germany (FRG) and the German Democratic Republic (GDR) of 21 December 1972. The treaty regulated the relations between the two German states and their respective citizens. The government of Bavaria claimed that this treaty (and therefore the act ratifying it) was unconstitutional because it violated the reunification duty that the Basic Law dictated to all (West-)German authorities.

The BVerfG ruled that it did not, and in its reasoning it developed the famous thesis that the German Reich had not disappeared in 1945, but continued to exist. Because the Reich has no state authorities of its own, the FRG can act on its behalf, at least on its own territory. Because of that, the Federal Republic shared an identity with the German Reich for its own territory and the Germans living there; FRG and German Reich are therefore not identical, but they are partly so (teilidentisch).Footnote ³⁵ Because of the continued existence of the Reich that was created in 1871, the GDR is not a separate country. Like the Federal Republic, the GDR also belongs to the German Reich and continues to be teilidentisch with it, at least until the German question has been definitively solved.Footnote ³⁶ This treaty is therefore a treaty under international law, the BVerfG argues, because under international law, both the FRG and the GDR are states; but that does not mean that this treaty creates a situation where both German states are subjects of international law per se. From the perspective of the FRG, the GDR is not a foreign country and the Grundlagen treaty does not change that. After laying the groundworks, the court continues in stating that even within a federal constitutional framework like the German one, international law plays a part in regulating the interstate relations; it is therefore perfectly conceivable that the same is true for the inter se relations of the two German states.Footnote ³⁷ As the quote from the decision shows clearly, the BVerfG explicitly mentions the 1927 Danube decision of the RStGH to show that there is in fact room for the use of customary international norms within the German legal order, although it had explicitly rejected this only a few months earlier. Since this decision of the summer of 1973, the BVerfG has not ruled on the use of CIL; its most recent decision, therefore, does seem to acknowledge the idea that the case law of the RStGH still has relevance today. And even if one would stick with the Coburg decision, materially speaking the same norms are applied: and the RStGH itself had already felt entitled to interpret customary norms in such a way that they would ‘fit’ within a domestic legal order with its own characteristics. The question if the relations between the German states are regulated by transformed customary norms of international law or by unwritten rules of German constitutional law falling under the scope of Bundestreue is therefore mostly academic in nature. Still, the final word – for now – from Germany’s highest court seems to be to underline the idea that CIL might have its part to play in regulating the relations between the German states.

4 The Use of CIL in the Republic of Austria

The Austrian federal constitution, the Bundes-VerfassungsgesetzFootnote ³⁸ or B-VG of 1920 contains a separate clause on the position of CIL within the Austrian domestic legal order. Article 9 (1) B-VG is more or less a copy of Article 4 RV.Footnote ³⁹ The Austrian doctrine is not quite clear on the exact hierarchical positions of these customary rules of international law within the Austrian legal order. The majority position seems to be that Article 9 B-VG contributes to these norms a position equivocal to the one explicitly provided for by Article 25 GG: superior to ordinary acts, but inferior to federal constitutional law.Footnote ⁴⁰ It has never played a large part within the domestic legal order of Austria, however.

This has a lot to do with the introduction of another provision into the constitution. This article, Article 15a, was introduced in the B-VG in 1974.Footnote ⁴¹ It regulates primarily that the federation and the states can enter into treaties with one another on matters of common competence. On the side of the federation, the federal government or a federal minister shall be competent to enter into the treaty; if matters of a legislative nature are regulated by the treaty, it shall need the approval by the federal legislator (1). Interstate treaties are only possible with regard to matters pertaining to the competences of the states themselves; they shall be brought to the attention of the federal government (2). For both treaties between the federation and one or more states and interstate treaties, the fundamentals of international treaty law shall be applicable. For interstate treaties, this applicability can be overruled by corresponding constitutional state acts of the states involved regulating otherwise (3).Footnote ⁴²

The new Article 15a was introduced into the federal constitution because of the lack of clarity with regard to the norms applicable on treaties and on interstate and federal-state relations within the Austrian legal order.Footnote ⁴³ There was never any doubt as to the nature of these kinds of treaties: they are as such not of an international, but of a domestic nature.Footnote ⁴⁴ Article 15a under 3 B-VG therefore transforms the fundamental principles of international treaty law into Austrian constitutional law and the article prescribes the use of these norms for all treaties between the federation and one or more states and between two or more states – with the exception of a regulation through corresponding constitutional state acts for the latter. Where both the 1919 Federal Constitution and the 1949 Basic Law are silent on the subject and the relevant German courts have never unequivocally stated that, indeed, norms of international law can and must be used as such within the domestic legal order, the Austrian constitutional legislator has taken sides. It has deemed all generally applicable norms of international treaty law such ‘fundamental principles’.Footnote ⁴⁵ This means that – generally speaking – the norms laid down in the Vienna Convention on the Law of Treaties (VCLT) are the ones applicable within the domestic legal order of Austria as well for the creation, interpretation and termination of treaties and the settling of disputes arising from them. The norms of the VCLT are for the most part codifications of existing rules of CIL. Both the constitutional legislator itselfFootnote ⁴⁶ and the Austrian constitutional court (Verfassungsgerichtshof, VfGH)Footnote ⁴⁷ have ruled that the convention norms should be applied to domestic treaties in the sense of Article 15a under 3 B-VG.

The 1998 decision by the VfGH is the only one in which a clear ruling is given on the application of the VCLT itself on domestic treaties. The case involved a treaty between the federation and the state of Vienna, however. Since Austria is a party to the VCLT, it can be argued that the treaty itself is applicable to the federal authorities, and therefore is applicable to a treaty between the federation and a state. But is the VCLT itself applicable to interstate treaties as well? This is somewhat questionable – the Austrian statesFootnote ⁴⁸ are not themselves parties to the VCLT, and the fact that the contents of the convention are transformed through Article 15a under 1 B-VG does not necessarily mean that the VCLT as such is transformed as well: the fact that the states are allowed to digress from it through state constitutional acts of their own also suggests that this might not be a straightforward situation. It could therefore perhaps be argued that on an interstate level, the VCLT norms are applicable in their ‘older guise’ as rules of CIL. The difference is, of course, highly theoretical. Only when a case is decided by the constitutional court about a pure interstate treaty (of which there are not many in Austria) will we perhaps know for sure what the formal status of these rules within the Austrian legal order is. So far, this has not happened. What is clear, however, is that Austria unequivocally chooses norms of an international nature, whose origins are undoubtedly customary in nature, to regulate interstate relations, showing the usefulness of these norms for that purpose.

5 Conclusion

This article has focused on the use of norms of CIL for the regulation of interstate relations in two states with a federal constitutional makeup, the FRG and the Republic of Austria. In Germany, the possibility of the use of CIL became relevant after the Great War, with the introduction of the new republican constitution in August 1919. One of the novelties of this new constitution was its Article 4, which regulated that general rules of international law (and therefore norms of CIL) were a binding part of German federal law. The newly minted Staatsgerichtshof für das deutsche Reich, whose main task it was to solve public law disputes of a federal nature, was quick to acknowledge the possibilities of this new provision in the constitution. In a number of cases on disputes between German states in the 1920s, it argued that when domestic constitutional norms did not provide a reasonable settlement, rules of CIL could be used via the way of Article 4 RV to solve the issue. The court did, however, interpret these norms so as to fit into a domestic legal order, which raises the question whether the RStGH used ‘real’ norms of CIL – or merely saw them as a source of inspiration to draw up what are in fact unwritten norms of constitutional law.

This ambiguity concerning the use of customary norms survived the German apocalypse of 1945. The post-war Bundesverfassungsgericht is partly the legal successor to the old Staatsgerichtshof. In three of its cases, it dealt with the question whether rules of CIL could play a part within the domestic legal order of Germany. An analysis of these cases shows that there is no clear-cut answer to the question whether the BVerfG acknowledges an independent role for customary norms in the German federal makeup.

Like Germany, Austria too has a constitutional norm transforming customary international norms into the domestic legal order. This article is no longer relevant for the questions dealt with in this chapter however, since the Austrian constitution also contains a specific norm dealing with the relevance of international law for interstate relations. Article 15a B-VG under 3 states since 1974 that rules of international law are relevant for the creation, interpretation and termination of treaties between the Austrian federation and one or more states and between two or more states. They can also be used to solve federal-state and interstate disputes. In the latter cases, the states party can decide to create differing norms through mutual constitutional state acts. The article was introduced into the Austrian constitution precisely to terminate the debate that is so topical in Germany: whether there is an actual role for international law in the regulation of interstate and federal-state relations. The Austrian constitution explicitly affirms this concept. The only remaining unclarity is if these norms are simply those of the Vienna convention or if they are of a customary nature. Since the Austrian states are not themselves parties to the Vienna convention, the latter might be the case. The Austrian VfGH has so far not ruled on this question.

The example of these two countries shows that international law norms can play a limited, but useful role within the domestic legal order of a federal state. Whereas in Germany an ambiguity has continued to exist on the question whether such norms, especially if they are of a customary nature, are valid as such within the domestic legal order of Germany or if they merely provide material inspiration for the development of unwritten domestic norms, such as federal loyalty, in Austria the constitutional legislator has been quite clear. Both federal-state and interstate relations in Austria are regulated by international norms, although these legal relations themselves are clearly domestic. Especially the VCLT is a relevant source of law in this respect. The ambiguity in Austria is of a different nature: it is not entirely clear whether the VCLT itself or materially similar norms of a customary nature are applicable in interstate relations, because the nine Austrian states are themselves not party to the convention. Despite this slight ambiguity, the introduction of a new article into the Basic Law along the lines of Article 15a B-VG, but instead focusing on rules of customary law, would seem a good idea to end the continuing vagueness in Germany resulting from the hesitant case law of the relevant German courts since the 1920s.