Drawing on the historical insight of Emer de Vattel to build on the contemporary arguments of Michael Walzer and David Luban, this article develops a model of sufficient risk as a necessary condition for anticipatory war to be deemed self-defence. This model holds that an anticipatory war may constitute legitimate self-defence (as opposed to aggression) when it aims to forestall a threat that poses a sufficient risk to the anticipating state. This is the point where a threat is both sufficiently (...) likely to materialise and sufficiently large to pose a grave risk. Due to crucial problems with the imminence condition for self-defence, I propose that the sufficient risk condition subsume that of imminence. The power of this model lies in its ability to encapsulate all factors raised by previous authors that are morally relevant specifically for anticipatory wars and categorise them as contributing to the judgement of the likelihood of the threat materialising and/or the magnitude of the potential threat. This parsimonious categorisation increases the accuracy and clarity of the moral theory on anticipatory war, making it more robust to change in the types of threats faced by states, and less ripe for manipulation to justify immoral wars. (shrink)