The European Union’s Participation in the Creation of Customary International Law and Its Impact on Member State Sovereignty

(Series Information) European Papers - A Journal on Law and Integration, 2023 8(3), 1645-1661 | Article | (Table of Contents) I. Introduction. – II. Law-making capacity as an expression of sovereignty. – III. The participation of international organizations in the formation of customary international law. – III.1. Overview. – III.2. Whose practice? – III.3. Which norms? – III.4. Conclusion. – IV. The practice of the EU and its relevance in the creation of customary international law. – IV.1. Overview. – IV.2. Legislative practice of the Union. – IV.3. Judicial practice of the CJEU. – IV.4. Positions taken by the Commission in (quasi-)judicial proceedings. – V. Conclusions. | (Abstract) This Article argues that the ability of the European Union to participate in the creation of customary international law curtails the sovereignty of its Member States. First, it shows that authority to participate in norm-creation constitutes a core aspect of sovereignty under international law. Second, it argues that the conduct of the European Union (as an international organization) may be determinative in ascertaining the existence and content of customary norms. However, that authority lacks an explicit basis in the treaties. Third, it asserts that this encompasses norms that are directly relevant for the Member States, potentially in circumstances outside of the scope of EU law. The Article then specifically discusses three types of acts of the Union and their relevance for the creation of customary international law, while providing examples that touch upon traditional inter-states relations. In particular, this concerns the legislative practice of the Union, the judicial practice of the Court of Justice of the European Union (CJEU) and public statements made by the Commission in (quasi-)judicial proceedings.