The use and choice of hard and soft law in international governance has been the subject of ever-increasing scholarly interest. This law and social science literature has primarily assessed the relative strengths and weaknesses of hard- and soft-law instruments as alternatives for international governance, as well as how these instruments can be combined as mutually reinforcing complements to lead to greater international cooperation over time. By contrast, we argue that under certain conditions, hard and soft law can and do operate as antagonists. In short, states and non-state actors increasingly use soft law not to “progressively develop” existing hard law, but to undermine it. In our previous scholarship we have demonstrated this antagonistic interaction of hard and soft law in the economic realm, where the international trade system often interacts in antagonistic ways with related areas of international environmental and cultural law. In this Article, we look beyond economic law, examining the interaction of hard and soft legal instruments with respect to two fundamental questions of international security law: (1) the legality of the threat or use of nuclear weapons, and (2) the legality of the use of force in humanitarian intervention under the “responsibility to protect” doctrine. In both cases, states and non-state actors have employed hard and soft law, not as complements in a progressive process of international legal development, but as antagonists, with soft-law pronouncements being used to undermine long-standing hard-law norms. In both cases, the result has been to obscure, rather than to clarify and elaborate, the most fundamental norms of the international legal system.