In the Gabčíkovo-Nagymaros proceedings, the parties, viz. Hungary and Slovakia, defended their conduct, amongst others, with arguments derived from the relationship between the law of treaties and the law of state responsibility, and from the law of state responsibility itself. In its judgment, the International Court of Justice disentangled the mixture of arguments derived from the law of treaties and the law of state responsibility advanced by Hungary, and drew a clear line between these two branches of international law. Second, it rejected several circumstances that were advanced by the parties to preclude the wrongfulness of their conduct. On both these accounts, the author opines that the declaratory dicta of the Court have contributed to the development of the law of state responsibility. Third, the Court decided on the legal consequences of the intersecting internationally wrongful acts committed by Hungary and Slovakia. According to the author, the Court erred in its reasoning on this account by confusing the award of cessation of the internationally wrongful acts with the award of reparation for these acts.