Is war an institution of international society and how is it constituted as such across the evolution of international society? This book is an inquiry into the purpose of war as a social institution, as originally put forward by Hedley Bull. It offers a comprehensive examination of what is entailed in thinking of war as a social institution and as a mechanism for order. Since the terrorist attacks of 9/11 the subject of war has become increasingly relevant, with questions about who can wage war against whom, the way war is fought, and the reasons that lead us to war exposing fundamental inadequacies in our theorisation of war. War has long been considered in the discipline of International Relations in the context of the problem of order. However, the inclusion of war as an 'institution' is problematic for many. How can we understand an idea and practice so often associated with coercion, destruction, and disorder as contributing to order and coexistence? This study contends that an understanding of the core elements that establish the character of war as an institution of modern international society will give us important insights into the purpose, if any, of war in contemporary international relations. This ground-breaking book will be of strong interest to students and scholars of international relations, international relations theory, the English school, security studies and warfare.