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 Vienna Convention on the Law of Treaties 1969 contains the body of rules governing the law of treaties. Its scope covers treaties between states. The rules largely reflect customary international law. So even where states are not parties to the Convention, its rules will often apply to their treaty relations by virtue of customary international law. Treaties between a state and an international organisation, or between international organisations, are not covered by the 1969 Convention. But international organisations may be party to the separate Vienna Convention of 1986, which adapts the rules of the 1969 Convention to international organisations. If they are not parties to the 1986 Convention, customary international law will apply to their treaty relations. The 1969 Convention does not cover oral agreements, which are in any case extremely rare. The 1969 Convention does not have retrospective effect. The implications of this are discussed.

This chapter explains the Court’s significant role as the ‘master of the sources’ of international law. The author considers what he defines as the ‘repressive dimension of Article 38’, before turning to consider the ways in which the Court has concretely used its role in respect of the sources of international law. Finally, the chapter critically examines how repression and mastery can often work together in international legal thought.

This chapter expands on the concept of reciprocity, looking at how it can be defined, its key characteristics, and the three key functions it plays: in norm creation, as a condition, and in execution of the law. It examines the defining elements of reciprocity: proportionality, relativity, equality, and interdependence. The chapter goes on to examine the nature of reciprocity, and its relationship to custom and general principles, two sources of international law. It illustrates how it is reciprocity’s relationship with the structural characteristic of sovereign equality in international law that explains many of its roles and defining characteristics.

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’.

This chapter studies the methodology used by the CJEU to identify CIL. In addition, it assesses whether that methodology is similar to, or differs from, that which other international courts and tribunals use (based on the two-element approach). The chapter argues that the notion of consensus can assist the CJEU in the identification process of CIL. In particular, two sets of cases are examined. First, those in which the CJEU has dealt with the interaction between the Resolutions of the UNGA and CIL in the identification process. And secondly, cases where general principles of public international law have been used to identify CIL. Resorting to public international law concepts (consensus) and techniques (external references to the PCIJ or the ILC) to identify CIL does not prevent the CJEU from adopting a particularist approach. The judicial practice analysed highlights the tension converging when the CJEU exercises its jurisdiction. A push towards autonomy (or particularism) can be noted on the one hand. On the other hand, the CJEU cannot avoid its nature as an international tribunal. The chapter argues that it is precisely for this reason that the CJEU resorts to a ‘flexible consensualist approach’ when faced with the identification of CIL thus allowing some room for cross-fertilization between international courts and tribunals.

The EU is often described as a sui generis international organization. While this is an apt description in some ways, it is far from self-evident that the special features that the EU exhibits are or should be part of the explanation of why and how it is bound by customary international law. This chapter seeks to show that, in the eye of public international law, the EU is not yet in a category of its own which would justify or otherwise affect the applicability of custom to it. It does so by investigating whether a special category of ‘regional economic integration organizations’ (REIOs) has emerged to which international law grants ‘preferential’ treatment, with a focus on three rules that were unsuccessfully proposed to cater for the specificities of the EU in codification projects carried out by the UN International Law Commission. The chapter also considers doctrinal and normative reasons why calls to distinguish the EU from other international organizations under customary international law have fallen on deaf ears, while recognizing the role that the EU has played in widening the horizons of the law of international organizations.

This chapter aims to examine a topic that up to now has received scant, if any, attention: the use of customary international law (CIL) by the European Parliament, the Council and the European Commission. To that effect, the actions resulting from the exercise of different powers of those three institutions (such as proposals, acts of secondary law, parliamentary questions, statements before the CJEU and other courts) have been taken in consideration. The examination of the practice of the political institutions of the EU does not reveal significant differences between them. References to rules of CIL from their side are relatively rare and mostly included in instruments with an international dimension. They all tend to invoke precise rules of CIL when doing so is essential for substantiating their legal position. They also seem to be aware of the fundamentals of this source of international law. By contrast, the European Parliament, the Council and the Commission fail to demonstrate (here is another common feature) that the rules of CIL they invoke are well established or that practice and opinio juris, as constitutive elements of CIL, concur. It must be highlighted as well that no pattern has been identified in this empirical analysis on the use of CIL by the political institutions of the EU.

Much of the debate about the EU and customary international law addresses the conditions under which customary international law is applied within the EU legal order, such as when it is used to challenge the validity of EU acts. To what extent can the EU also contribute to the development and identification of customary international law? This chapter argues that the EU is not only bound by customary international law, but can also contribute to the development and identification of customary law in its own right. It examines these questions in light of the International Law Commission’s 2018 Draft Conclusions on Identification of Customary International Law. It first discusses some of the conceptual issues faced by the ILC and some of the ways that the EU may contribute to the development of customary international law through its practice and opinio juris. The chapter then turns to the role of the Court of Justice of the European Union and the role it plays in the development and identification of customary international law. It argues that EU practice is relevant when determining rules that apply to the relationship between the EU and third states and organizations.

This chapter discusses how customary international law enters the sphere of EU law and to what extent it determines the relationship between the EU institutions, its member states and individual persons. Against this background, it is examined how customary international law is integrated into the EU legal order, what status it enjoys within that legal order, how it can be applied by EU institutions and whether it can serve as a benchmark for judicial review. Assuming that EU law is an autonomous domestic legal system, these questions are discussed from the perspective of EU constitutional law. Here it is argued that the Court of Justice of the European Union, in evaluating these issues, focusses too much on the idea of the autonomy of EU law. In order to provide provisions of customary international law with practical effect the validity of EU law must also be able to be reviewed on the basis of international law standards.

The EU functions as a receptor and promoter of customary international law (CIL). However, from a theoretical (and international law) perspective, the questions why and when the EU is bound by CIL still have not been answered in definite terms. This chapter argues that the EU should be afforded a similar treatment as is afforded to (newly independent) states. This is based on a close examination of its practice in the field of international humanitarian law (IHL), where the primary organization tasked with ensuring peace and security – the United Nations – has so far avoided explicitly affirming that it is bound by customary IHL. After showing that it is indeed the EU which might bear direct responsibility in the context of military operations authorized under the Common Security and Defence Policy, the chapter examines the EU’s behaviour when it comes to new regulatory efforts, its executive and operational conduct, and the CJEU’s case law. On the basis thereof, it is concluded that the EU’s practice confirms the theoretical conclusions reached, and that therefore any theoretical uncertainties on whether and why international organizations are bound by CIL might in future be answered by paying closer attention to their practice.