The proliferation, globalization, and fragmentation of law in world politics have fostered an attempt to re-integrate International Law (IL) and International Relations (IR) scholarship, but so far the contribution of realist theory to this interdisciplinary perspective has been meagre. Combining intellectual history, the jurisprudence of IL and IR theory, this article provides an analysis of John H. Herz’s classical realism and its perspective on international law. In retrieving this vision, the article emphasizes the political and intellectual context from which Herz’s realism developed: the study of public law in Germany during the interwar period and in particular the contribution of Hans Kelsen and the pure theory of law to the study of international law. Herz was deeply inspired by Kelsen but he criticized the pure theory for ignoring the sociological foundations of law. Following his emigration to the United States, Herz embraced realism but without disregarding international law. Indeed, his mature, globally oriented realism offers a balanced, fruitful perspective for thinking about the relationship between politics and law that is deeply relevant for contemporary theory: it challenges modern, law-blind variants of realism and holds considerable potential for contributing to the approaches that have most successfully studied the law–politics nexus.