 If customary international law is made of practice, what happens when the practice of states is in breach of custom? Are breaches of a customary rule to be considered as wrongful acts or as amounting to a new practice which displaces and changes the former customary rule into a new different one? This question relates to the normativity of customary rules. If customary rules only reflect practice, they are not much different from reality and they change according to reality. Translated in the field of international relations, this would mean that law and power are identical. Law would just be an apology for power. But if law and reality are the same, there is not really any law. Law is not normative. On the other hand, if rules are completely disconnected from reality, they would be very normative but they would lack any effectiveness. Law would be purely utopian and just words. According to Marty Koskeniemi, who is professor at the University of Helsinki and the leading scholar of international law, according to him, this tension between apology and utopia is inherent in international law and always present in arguments about international law. The tension between how things are, apology, and how things should be utopia, is indeed reflected in the balance between the two ingredients of customary law, that is practice and opinionaries. This balance is difficult to strike in order to assess the emergence of the customary rule but it is even more at play when the question is the one I started this video with. What happens when the practice of states is in breach of an existing custom? Is the customary rule changed and displaced according to the new practice? The issue of the survival of a customary rule, despite a pattern of practice which contradicts it, has been addressed by the International Court of Justice in a famous case between Nicaragua and the United States of America and please turn to the next reading to find out more about it.