The present chapter examines whether, and if so to what extent, customary international law applies within the internal legal sphere of the EU, that is, between EU member states within the EU law’s scope of application as well as in their legal relations to the EU. In essence, the relevance of customary international law within the EU’s internal legal sphere is about the EU’s assertion of autonomy and self-containment that has been unfurled by the CJEU. The analyses of key areas of customary law (e.g. diplomatic relations, sovereign immunity and equality, rules of responsibility) reveal a complex picture of the its rules’ relevance within the EU. They play a more tangible role in the relationship between EU member states, first and foremost in sovereignty-related areas, than in member states’ legal relationships to the EU. Nevertheless, the present chapter shows that despite being a ‘new legal order’, the EU treaties still constitute a subsystem of public international law, albeit one which manifests typical characteristics of self-containment.

Treaties rely on the assumption that certain circumstances, essential to the conclusion of the treaty, will remain unchanged. It is on the basis of those circumstances that parties reach an agreement and pacta sunt servanda aims to safeguard those shared expectations. Still, States can be freed from their contractual obligations through peaceful means, when circumstances leading to the conclusion of a treaty change and obligations under a treaty become unduly burdensome. An unforeseen fundamental change of circumstances can be invoked as grounds for terminating a treaty if it affects the essential basis of the treaty and radically transforms obligations still to be performed. These conditions can all be met when coastlines or ecological circumstances undergo fundamental changes, affecting the essential basis of maritime delimitation agreements or related agreements concerning resource management. However, treaties establishing boundaries are immune from termination or revision on these grounds. Still, such agreements can be reinterpreted as a result of fundamental, environmental changes and this exception arguably, only excludes maritime boundaries of the territorial kind.

Chapter 2 provides a contextual discussion of why some non-States Parties object to the jurisdiction of the ICC. The chapter engages with the allegation that the Rome Statute infringes on the sovereignty of non-States Parties by allowing the ICC to prosecute their nationals in certain circumstances. This involves an analysis of how the Statute affects non-States Parties and an evaluation of whether such effects amount to an infringement of State sovereignty. How sovereignty interacts with international law is largely a matter of perspective, and how States perceive sovereignty shapes their view of whether there is an acceptable legal basis for the Rome Statute’s jurisdiction provisions. This chapter argues that the ICC needs to recognise such concerns and formulate the legal basis for its jurisdiction in a way that maximises the role of State consent.