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.

The chapter highlights the central role national jurisdictions (should) play in the system of international criminal enforcement and addresses the most common legal issues and practical obstacles which may obstruct the pursuit of accountability at the domestic level. The chapter provides an overview of the relevant state practice from the earlier notable precedents to the most recent instances of prosecution and adjudication of core crimes before domestic courts, in particular under the universal jurisdiction. It clarifies the scope of the duties international law imposes on states, including the obligation to extradite or prosecute. The chapter then zeroes in on every principal issue related to the domestic prosecution and adjudication of international crimes, such the need for adequate implementing legislation as well as the extent to which domestic prosecutions may be hindered by the statutes of limitations, the prohibition on retroactive application of penal provisions, and the principle of ne bis in idem (double jeopardy). The chapter’s final section addresses the political and practical obstacles to tackling impunity for international crimes at the domestic level.

The Introduction outlines the nature of the study and the rationale for undertaking it. It argues that collective self-defence remains under-theorised (and collective self-defence practice under-analysed) despite the increased invocation of it by states in recent years. The Introduction also discusses the book’s methodology, particularly in relation to its focus on state practice and opinio juris as crucial ‘raw materials’ for legal standards in this context. The key facts and findings of the famous 1986 Nicaragua decision of the International Court of Justice are then summarised, because the case is referred to throughout the book. The Introduction concludes with a summary of the structure of the book.

In Chapter 3, ‘The Proliferation of Indigenous Peoples’ Participation, 1982–2007’, I trace how a pattern of state practice unfurled over a span of 25 years, beginning with the establishment of the Working Group on Indigenous Populations (WGIP) and spreading to numerous other UN organs, international organizations, and other international governance mechanisms. It at first takes a chronological approach, tracing relevant practice from 1988 to 1994 in processes such as the negotiations towards the International Labour Organization Convention No. 169 on Indigenous and Tribal Peoples, UN conferences, and several UN expert workshops. It then considers how momentum grew towards the establishment of the UN Permanent Forum on Indigenous Issues. The chapter next turns to consider several parallel trends. It turns to mechanisms for Indigenous peoples’ participation in environment and development negotiations, before tracing Indigenous peoples’ participation in decision-making in international development institutions. The chapter ends by examining three regional stories: the Arctic, Latin America and the Caribbean, and the European Union.

The chapter is based on a survey of state practice of CIL identification in connection with execution and annulment proceedings. The chapter proceeds in three main parts. Part I examines the nature of state practice in the form of judicial decisions, addressing, among others, the question of whether and to what extent a decision by a domestic court may be seen separately or concurrently as constitutive of practice, in the terms of ICJ Statute Article 38(1)(b), and/or as a subsidiary means, within the meaning of ICJ Statute Article 38(1)(d). Part II provides an overview of selected features of the surveyed body of state practice. This overview shows how domestic courts and states parties to execution and annulment proceedings before those courts approach CIL identification and rely on ICJ Statute Article 38(1)(a)-(c) to ascertain whether a certain source does constitute one of the sources of law recognised in those subparagraphs of ICJ Statute Article 38. Part III investigates the significance of the surveyed state practice, with a particular focus on some of the wider implications this state practice might have with respect to broader debates on CIL identification and the sources of international law, and shows a bi-directional interaction between general international law and international investment law.

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.

The general continuity of investment treaties stands in contrast to traditional rules, according to which the outbreak of war abrogated treaties between belligerents. Even today, it remains a common narrative that specialised customary rules exist on the (dis-)continuity of treaties at the outbreak of armed conflict. Chapter 2 critically scrutinises and ultimately rejects this doctrine of the effects of armed conflicts on treaties. While investment treaties also continue to operate under the International Law Commission’s ‘Draft Articles on the Effects of Armed Conflicts on Treaties’, remaining uncertainties as to the state of the law render a comprehensive examination necessary. The chapter argues that the Draft Articles and the doctrine they incorporate suffer from severe shortcomings. The better approach towards the effects of armed conflicts on treaties and more appropriate reading of contemporary international law, the chapter submits, is to discontinue the traditional invocation of specialised customary rules and reorient the debate exclusively toward general treaty law and the Vienna Convention on the Law of Treaties.

This chapter provides an original inquiry into the use of General Assembly resolutions within domestic legal systems. The scholarly literature is silent on this, being entirely focused on the use of resolutions by international courts and tribunals. As a result, this chapter supplements the current academic debate by revealing a neglected aspect of the legal significance of resolutions. It evaluates the degree of legal significance that states attach to resolutions at the implementation stage by analysing the process of incorporation of resolutions into domestic law. It shows that the source of the legal significance of resolutions can be ultimately traced back to the will of individual states to rely on resolutions depending on context and circumstances. As a result, generalizations about the legal significance of resolutions within domestic legal systems appear to be neither convincing nor useful.

This chapter examines the different functions that resolutions serve in state submissions before international courts and tribunals. The analysis revolves around two interrelated issues. On the one hand, it discerns between instances in which states treat resolutions as evidence of state practice and instances in which states treat resolutions as the practice of the General Assembly. On the other hand, the analysis evaluates the extent to which states and judges hold different views on the legal significance of resolutions. It demonstrates that issues relating to the legal significance of resolutions revolve around a double dichotomy generated by the different positions taken by states and judges in relation to assertion and attribution of the source of the legal significance of resolutions to state practice or the separate will of the General Assembly. It contends that it is difficult, if not impossible, to generalize about the legal significance of resolutions given the character of international polyarchy of the General Assembly. It then offers insights into the concept of autonomy of the General Assembly from its member states.

This chapter provides an innovative analysis of the concept of legal significance of resolutions. It evaluates the common traits of state practice related to the attribution of legal significance of resolutions in relation to three contexts – at the adoption stage, within domestic law and in international practice. Subsequently, it evaluates whether it is possible to predict the presence of legal significance of resolutions by providing a critique of the existing tests of legal significance devised by scholars. It concludes by arguing that there is a significant discrepancy between the findings of this study and scholarly positions on the normative value of resolutions. The latter tend to focus on the legal significance at the adoption stage, thus underplaying the relevance of state practice as a signpost of legal significance at the implementation stage – both domestic and international.

The scholarship on sources of international law has not satisfactorily answered these questions: are sources regulated by any ‘rules’? Are ‘rules on sources’ part of international law? What is the source of ‘rules on sources’? The standard answer to these questions is that sources are regulated by state practice reflected in Article 38(1) of the Statute of the International Court of Justice. This position, albeit professing to be practice-based, does not account for the existence of ‘rules on sources’, nor for their legality and source-based character. Other practice-based positions have also proven insufficient: some, postulating the legality of rules on sources, fail to address their source-based nature; others, affirming both the legality and source-based character of rules on sources, and invoking custom as source thereof, fail to substantiate their claims, which are merely asserted or based on decisions of early arbitral tribunals, thus assuming custom to be judge-made (a ‘custom in foro’). This chapter examines the suitability of custom as source of rules on sources and proposes a shift away from custom in foro towards customary international law proper, established by state practice invoking ICJ Statute Article 38(1) outside ICJ proceedings, major instances of which it surveys.

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.