This article considers the relationship between geopolitical change and the evolving international rules on military force. Its focus is the impact of the United States’ rise to hegemonic status on the rules governing recourse to force (the jus ad bellum) and the conduct of hostilities (the jus in bello, otherwise known as ‘international humanitarian law’). For reasons of space and clarity of analysis, the article does not focus on the different, more traditional IR questions of whether and why the behaviour of the United States might be constrained by these rules.