Customary International Rules Addressed to Member States and EU. Mapping Out the Different Coordination Models

Practice shows the existence of complex legal situations in which customary international rules applicable to the Member States interfere, even indirectly, with the competences of the Union, and vice versa. On the one hand, the implementation of a rule of customary international law by Member States could affect rights and obligations under EU law. On the other hand, the exercise of EU competences could affect the rights and obligations conferred on Member States by customary law. In these situations, the Union must reconcile two “equal and opposite” needs. On the one hand, it must ensure that Member States’ exercise of rights and obligations under customary international law does not undermine the effectiveness of EU law. On the other hand, it must prevent EU competences from interfering with the rules of customary international law applicable to the Member States. This Article aims to explore how the Union reconciles the exercise of EU competences with the exercise of Member States’ competences under customary international law. After examining the most prominent models that could theoretically be used to coordinate the two spheres of competence (section II), the attention will turn to the approach adopted by the ECJ (section III) to determine whether this approach affects the prerogatives of the EU Member States as sovereign states under international law (section IV).