Hans J. Morgenthau’s approach to international relations has been the subject of a number of studies since the initial publication of his works. Deeply insightful, this literature has offered valuable commentaries on the wider implications of this theorist’s works, which focus on a deeply varied number of topics and debates. Through an examination of Hans J. Morgenthau’s wider works, this paper contributes to this literature, offering an analysis of his theory of international law. It explores the elements of his theory of international law, arguing that this aspect of his theory of international relations, though explored less frequently in the wider literature, remains of great relevance today. Finally, it argues that, though often misunderstood, Morgenthau’s classical realist approach provides a powerful commentary on the fundamental problems faced by international law; something which remains particularly relevant in relation to the problems faced by international law, and international disarmament and arms control treaties today.