This chapter examines how war fits into the study of international relations and the ways it affects world politics. It begins with an analysis of the work of the leading philosopher of war, Carl von Clausewitz, to highlight the essential nature of war, the main types of war, and the idea of strategy. It then considers some important developments in the history of warfare, both in the West and elsewhere, with particular emphasis on interrelationships between the modern state, armed force, and war in the West and in the global South. Two case studies are presented, one focusing on war and Eurocentrism during the Second World War, and the other on the impact of war on society by looking at France, Vietnam, and the United States. There is also an Opposing Opinions box that asks whether democracy creates peace among states.