The ethics of peace and war is central to the idea of world ethics. In fact it can be argued to be one of the central ethical issues in international Relations more generally. This book looks at three general theories concerned with the role of ethics in international politics, focusing on the specific problems of war and armed conflict. The author explores the connections between these three theories - cosmopolitanism, reason of state, and just war theory and pacifism - and puts forward the argument for the appropriateness of cosmopolitanism (with its emphasis on the equal worth of all human beings as the basis for a global moral community) above the other two general theories in light of current issues concerning war and armed conflict, changing conceptions of peace and security, the prevalence of internal over international wars, and the increasing emphasis on international humanitarian intervention as a justification for the use of military force.