Republic of Slovenia v Republic of Croatia (C-457/18): "Am I my Brother's Keeper?" International Agreements by Member States and the Limits of the European Court of Justice's Jurisdiction

In Slovenia v Croatia the Court of Justice of the European Union had the opportunity to clarify the boundaries of its own jurisdiction over alleged breaches of EU law, when these derive from alleged violations of international agreements. It declined jurisdiction over claims that a Member States would have violated EU law, because these claims were merely ancillary violations to an international agreement whose subject matter fell outside of EU competence and was thus not part of EU law. In an important and cautious pronouncement over the relationship between international law and EU law, the Court formalised a doctrine of "ancillary violation" and showed self-restraint by limiting the scope of its jurisdiction when this would result, albeit indirectly, in an expansion over a matter falling within the exclusive competence of Member States.