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.

There has been discussion in recent years about ‘the end of treaties’ and how international cooperation is now often achieved through methods other than the formal treaty-making process. Over the past decade, states have sought to deal with global challenges ranging from financial stability to climate change and nuclear non-proliferation, not through binding international instruments, but through non-binding commitments, diplomacy and unilateral domestic measures. This has brought into doubt the continued relevance of treaties, which can be viewed as rigid and slow moving compared with other forms of international co-operation.

One of the significant changes in the landscape of international law in recent decades has been the increase in the number of international courts and other forms of international dispute settlement. Not only has the number of such bodies increased, but their role and function has changed significantly, and include compulsory procedures and may allow a role for individuals (and other actors) in a realm once dominated by inter-state procedures. The European Union has contributed to this changing landscape. The Court of Justice of the European Union has been a model for other regional courts.

When the European Communities were first established they were a form of ‘legal experiment’, in which international law was used as the building blocks of a new type of international organization. Discussions about the European Union from an international law perspective often looked at how the EU could be a model for the functioning of the international legal order. Cassese, for example, put EU law forward as a model for the international system, especially the way in which EU law can have supremacy over inconsistent national law.

The European Communities were first established and built using public international law instruments. Since its establishment, there has been debate about whether, and to which extent, the Community (now Union) transformed into something else – from an organization established under public international law into a new legal order that has since escaped these international law origins.

How is the European Union (EU) understood from the perspective of international law? Much of the discussion about the EU in international law scholarship has dealt with questions such as whether the EU can still be considered a ‘creature of international law’, and whether EU law should be considered international law. The question addressed here is different. It focuses on how the EU fits within the wider system of international law. The way that the EU is perceived and dealt with from the perspective of international law is legally relevant when the EU acts on the plane of international law.

How can the European Union contribute to the development and identification of customary international law? Alongside treaties, customary international law is a key source of international obligations; it has been described as ‘the main mode of international regulation’. Yet in comparison with the EU’s treaty practice, the EU’s contribution to the development and confirmation of customary international law has been the subject of less debate. There has been discussion about how customary international law applies in the EU legal order. The EU Court of Justice has held that ‘when [the EU] adopts an act, it is bound to observe international law in its entirety, including customary international law’.

The EU’s participation in international agreements has been the main method by which it acts on the international law plane. Yet the EU is also a participant in a growing number of international organizations (IOs) and international forums. Early in the life of the EU, it became evident that it was not enough that it participate in treaty making, but also engage with, and even participate in IOs. This chapter focuses on the legal issues that arise from the EU’s participation in IOs and other international forums.

This chapter explores another pillar of public international law: responsibility for internationally wrongful acts. In doing so, it examines how rules of international responsibility, especially those regarding the responsibility of international organizations, have been influenced by the unique nature of the EU and the EU legal order. The chapter begins by discussing and framing the debate about the responsibility of international organizations (Section 6.3). It briefly discusses some of the unique features of the EU legal order that may require the law of responsibility to be ‘adapted’ in the EU context.