The naturalness of warThucydides' History of the Peloponnesian War is one of the few foundational texts in international political thought. I introduce Thucydides' work and his influence on international relations theory and subsequent history. I consider his role as a theorist of realism and examine the nature of and basis of realism as the default perspective of international politics. Thucydides also provides an historical account of the collapse of Athenian democracy under the pressure of war, so this chapter explores the themes of democracy, strategy and leadership in wartime. Thucydides' account of a system of political communities (poleis) interacting in the absence of an overarching hegemonic power is a model of international order that continues to dominate international affairs and diplomacy. Its account of the dynamics of international politics is still thought by many to have lessons for present-day international politics and a changing world order.Prior to the first Gulf War in 2003 it was common to find commentators and scholars framing the debate about the war or its subsequent conduct through reference to the Greek historian Thucydides. Perhaps this is not surprising from classically educated journalists or academics writing 'op-ed' pieces, but references to Thucydides also extended into the western military itself. Thucydides