This chapter addresses the chronological paradox of customary international law (CIL). The paradox is that for a new customary rule to be created states must believe that the law already obligates the behaviour specified in that rule (opinio juris). However, the behaviour in question can only be legally required once that rule has been created. As a result, creating a new customary rule would be impossible, or at the very least an incoherent process. This chapter challenges this conclusion. In addressing the chronological paradox, it provides a coherent interpretation of the creation of new customary rules. It argues that the sense of legal obligation (opinio juris) emerges from the general principle of good faith. Good faith leads to legal obligations, which compel a subgroup of states to engage in specific behaviour. Then, as a result of this subgroup’s repeated behaviour, a new customary rule emerges, obligating the entire community of states to act accordingly. To explain the shift from good faith to legal obligations and from legal obligations to customary rules, the chapter draws on interpretivism, social ontology and contemporary research on constitutive rules.

Customary international law (CIL) is particularly vulnerable to the accusation that it is no more than ‘mere assertion’, a creation of the courts, if not downright fantasy. Yet it is in CIL that one finds the strongest claim to objectivity in international law. It is expressed in the doctrine that one of the elements of CIL is state practice, which represents the ‘objective’ element of CIL. It is thought to supplement the ‘subjective’ or ‘psychological’ element of CIL: opinio juris. This chapter argues that the notion of state practice as a set of ‘material facts’ that should be ‘identified’ and from which customary norms can be ‘induced’ is grounded in obsolete epistemology. The identification of state practice is more adequately described as a selection of what deserves to be counted as state practice. It is argued that the starting point for this selective process is opinio juris. Opinio juris does not come after the fact, as a subjective feeling of obligation that is superadded to a set of otherwise objective facts. Opinio juris is the indispensable conceptual framework without which habits and usages cannot even be ‘seen’ as state practice.

This chapter examines two ways the customary norm could have emerged post-1945: the two-element approach or that article 2(4) gave rise to a new customary rule of its own impact, following the approach of the ICJ in the North Sea Continental Shelf Cases. It demonstrates the challenges of applying the two-element approach to the customary prohibition of the use of force due to the presence of the parallel and near-universal treaty obligation in article 2(4) of the UN Charter, which makes it difficult to identify sufficient relevant State practice and opinio juris outside the treaty. Establishing evidence of the customary rule and its content in this way depends on a number of theoretical issues that remain unsettled or over which significant controversy exists. This chapter then applies the criteria set out by the ICJ in the North Sea Continental Shelf Cases to article 2(4) and argues that article 2(4) meets this test. This chapter concludes that, in contrast to the right to self-defence in article 51 of the UN Charter which explicitly has its origins in customary international law, article 2(4) is itself the origin of the customary international law prohibition of the use of force.

Chapter 4, ‘Emerging Status as Law? Participation of Indigenous Peoples in International Governance, 2007–2022’, continues the narrative from the point of the adoption of the UNDRIP in September 2007. In this period, the general trend of the proliferation of participatory policies and mechanisms identified in Chapter 3 only intensified. Organizations that had not previously attempted to include Indigenous peoples in a systematic way now adopted mechanisms and policies to do so, while those that had previously instituted such mechanisms or policies sought to enhance them. This chapter maps the contours of this continuing trend, showing how the participation zeitgeist spread through numerous UN organs and international organizations. It assesses the question of states’ and international organizations’ motivations for enabling Indigenous peoples to be heard in such fora. It shows how states and international organizations now increasingly justified participation policies with reference to the UN Declaration on the Rights of Indigenous Peoples. It demonstrates how in nearly all cases barriers to full and effective participation remain.

The author argues that ‘will’ and ‘consent’ are different. According to him, no State’s will is entirely free. However, this does not preclude its consent from being valid. State consent displays different shades of will: while unilateral acts are the epitome of ‘willing consent’, the degree of willingness required when accepting a treaty is weaker, until it almost disappears in the case of custom, or general principles of law. The author argues that opinio iuris and consent are also different notions: you may feel legally bound even if consent is very remote. However, whatever role ‘will’ plays in the formation of rules, once the rules exist, States are, according to the author, bound and their will is trapped. The author makes the argument that, if neither will nor consent explain the basis of a State’s obligation when it is no longer willing to implement it, they nonetheless have a stabilizing and legitimizing role. He argues that consent makes the acceptability of the obligation stronger, by comforting its legitimacy, which also makes its implementation more effective.

Anti-satellite weapons that rely on violent impacts and create space debris are regarded as a major threat to the exploration and use of space, including the use of space assets for communications and Earth imaging. This chapter examines two ways in which the testing of such ‘kinetic’ weapons might already have become illegal. First, the accepted interpretation of Article I of the Outer Space Treaty may be evolving as a result of the changing practice of the parties to that treaty. In short, many states are behaving as if tests of anti-satellite weapons that create debris are contrary to the ‘freedom of exploration and use of space’. Second, the same practice and an accompanying opinio juris may be contributing to the development of a parallel rule of customary international law. This chapter also examines the legality of the use of kinetic anti-satellite weapons, as opposed to their testing. Two additional, separate bodies of international law are relevant here: the jus ad bellum governing the recourse to armed force, which includes the right of self-defence, and the jus in bello governing the conduct of armed conflict. A close analysis leads to the conclusion that any use of a kinetic anti-satellite weapon would be illegal today because of the growing crisis of space debris.

This chapter is concerned with Germany’s perspective on the foundations and functions of international law. It is structured in two parts: international law in general and sources of international law. The second part examines Germany’s view of the Joint Comprehensive Plan of Action (JCPoA) that sought to limit Iran’s nuclear programme. It is argued that the political aims of Germany do not justify the twisting of international law. The Federal Government portrayed the document as a binding international treaty. It did so by speaking of an ‘agreement’, from which the US had decided to ‘withdraw’ and by depicting the participants as ‘parties’. Further, Germany argued that Iran is bound by ‘obligations’ under the JCPoA. Germany also spoke of ‘Articles’ in the JCPoA and asserted that the document was ‘signed’. Moreover, Germany argued that the JCPoA became international law through the endorsement of the UN Security Council. However, the UN Security Council never marked its endorsement as a binding decision. Four and half years after the adoption of the JCPoA, Germany admitted that the JCPoA constitutes only a record of mutual political commitments and is therefore ‘soft law’.

What is international law, and how is it different from domestic (national) law?  This chapter provides an answer to those questions and introduces other foundational concepts in international law such as opinio juris, sovereignty, and the state. We trace the development of international law from Western/European perspectives and discuss how international law was often experienced as oppression and violence in the non-Western world. The chapter concludes with a consideration of the classic sources of international law: customs, treaties, general principles of law, judicial decisions, and scholarly writings, each of which is presented in some detail.

The ‘cliché’ two-element theory of customary international law can provide a simple solution against the legion of alternative theories. As a manifestation of international law that does not directly spring from the ‘will’ or ‘consent’ of states, it reflects their perpetual international relations. States do not voluntarily form a will at the international level but consciously or unconsciously influence its creation through their actions. Following the metaphor of ‘Schrödinger’s custom’, until an observer is introduced to determine what the particular customary rule is in a certain moment, customary international law remains ‘mixed or smeared’. This chapter builds on the instructive value of realism when dealing with states, focusing on the interplay of their interests in the formation of customary international law. While it is obvious that parties bring such interests to the table when negotiating a treaty, it appears less obvious whenever scholars seek to harness custom for the normative project of international law. The chapter argues that more even than other sources, customary international law will most likely reflect an equilibrium of interests. Thereby, effectivity and reciprocity, as the catalysts of international law formation, guide the identification of the legal status quo.

This chapter begins by explaining why international lawyers typically begin discussions about the sources of public international law by referencing Article 38 of the Statute of the International Court of Justice (ICJ). It then introduces treaties and custom, which are the two main sources of law in this field, before discussing other sources, namely general principles of law, decisions of international organizations, unilateral declarations, as well as judicial decisions and the teachings of international legal experts. The chapter ends with a discussion of non-binding instruments, which do not contain binding legal rules, but are nonetheless significant in the international legal field, as they contain norms that impact the behavior of states.

This chapter begins by explaining why international lawyers typically begin discussions about the sources of public international law by referencing Article 38 of the Statute of the International Court of Justice (ICJ). It then introduces treaties and custom, which are the two main sources of law in this field, before discussing other sources, namely general principles of law, decisions of international organizations, unilateral declarations, as well as judicial decisions and the teachings of international legal experts. The chapter ends with a discussion of non-binding instruments, which do not contain binding legal rules, but are nonetheless significant in the international legal field, as they contain norms that impact the behavior of states.

Chapter 6 examines whether the weapons review obligation as formulated under Article 36 could have now attained customary international law status with the consequence that even States not party to Additional Protocol I are equally required to review weapons for their compliance with applicable international law. This issue remains contentious in the legal literature, with some commentators supporting the customary nature of Article 36 and others being less certain of this obligation. The chapter argues that, in the absence of ‘extensive and virtually uniform’ State practice showing that new weapons, means and methods of warfare are reviewed at the earliest stage in the acquisition process as a matter of law, no customary rule mirroring Article 36 exists. The chapter also demonstrates that, contrary to the argument advanced in the expert literature, an obligation narrower in scope – requiring States to review weapons ‘before fielding’ – also cannot be ascertained. Such an argument is simply not persuasive given the lack of required State practice and opinio juris. [164 words]

An increasing number of jurisdictions are using environmental impact assessment as a tool for climate change mitigation (in brief, ‘Climate Assessment’, or CA). This chapter debates whether this is a legal obligation, or whether this even makes sense. Benoit Mayer argues that CA is emerging as a rule of customary international law, and that, moreover, it is a potentially useful mitigation tool. Alexander Zahar, by contrast, questions the meaningfulness of CA, arguing that it is impossible to determine what constitutes a significant, excessive, or disproportionate emission of greenhouse gases in the case of a proposed activity subject to CA, or at all.

An increasing number of jurisdictions are using environmental impact assessment as a tool for climate change mitigation (in brief, ‘Climate Assessment’, or CA). This chapter debates whether this is a legal obligation, or whether this even makes sense. Benoit Mayer argues that CA is emerging as a rule of customary international law, and that, moreover, it is a potentially useful mitigation tool. Alexander Zahar, by contrast, questions the meaningfulness of CA, arguing that it is impossible to determine what constitutes a significant, excessive, or disproportionate emission of greenhouse gases in the case of a proposed activity subject to CA, or at all.

The adoption of the Treaty on the Prohibition of Nuclear Weapons (TPNW) at the United Nations General Assembly and its movement toward ratification raises both hopes amongst some and fears amongst others regarding the development of customary practice under international law. Although the TPNW purports to strive for universality, significant questions remain regarding the Treaty’s aims and achieving legal unity within the international legal order. The purpose of this chapter is to explore these and other issues regarding the TPNW’s prospects of achieving universality under customary international law and opinio juris.

Have International Investment Agreements (IIAs) generated or changed customary law? (1) Scholarship’s optimistic approach to the status of customary international law fails to distinguish the elements of custom-creation from their proofs. (2) IIAs are not instances of expropriation or non-expropriation, hence they cannot serve as state practice. Verbal acts are not practice of the content of that verbal act. (3) IIAs are not opinio iuris because it is unlikely that treaty parties express preferences regarding customary law by concluding a treaty. (4) If treaties are an opt-out from general international law, then IIAs are likely concluded to counter perceptions of custom and a fortiori not custom-building. Therefore, a customary international law on expropriation is unlikely to be shaped significantly by IIAs and even if they did shape it, the resulting norm would be highly unspecific.