The distinction between offensive and defensive force is central to modern international law. As developed in the interwar years and later codified in the United Nations Charter, international law has strictly banned resort to offensive force, the only possible exception being operations mandated by the Security Council. In the just war tradition (JWT), by contrast, some kinds of offensive force are permitted, including humanitarian interventions. This article compares these two different ways of understanding the distinction between offensive and defensive force. It suggests that there is a need to specify in which cases the offensive use of force may be legitimate for humanitarian purposes, and it does so by drawing on the just war idea of ‘reaction against wrong-doing’. The concept of a reactive use of force, inspired by St. Augustine and the subsequent JWT, is thus central to the article’s argument.