This article makes the case that realist international theory provides firm theoretical and conceptual foundations for thinking ethically about the 'war on terror'. It begins by outlining the fundamental political and theoretical assumptions underpinning the realist approach to ethics, arguing that realist theory is based on three core principles: prudence, scepticism, and reciprocity. Drawing on the political philosophy of Michael Oakeshott, Reinhold Niebuhr, E. H. Carr, and Hans Morgenthau, the article argues that the belief in universally valid ethical and political principles found in liberal and cosmopolitan approaches to international politics risks conflating particularist interests with universalist claims, and generating foreign and security policies based on a moral crusades rather than a hard-headed calculation of national interests. For realists, the multiplicity of political communities and rival conceptions of the 'good life' inherent in the anarchic structure of international politics necessitates an ethical approach rooted in communitarian political theory. The article concludes by outlining a realist response to some of the major ethical questions raised by the 'war on terror'.