Thucydides is generally regarded as the founder of Realism in IR because of his vivid descriptions of war and power politics. A strong Realist account rests on sharp dichotomies between domestic and international politics, power and justice, nature and convention. Reading Thucydides through these dichotomies is not limited to IR; in fact, most classical and philosophical scholarship on the historian is informed by this vocabulary. I argue that such readings cannot hold under close scrutiny because they turn Thucydides too much into a sophist. As closer attention to how nature and convention, power and justice, domestic and international are deployed in the History reveals, Thucydides is deeply concerned with moving beyond standard sophistic oppositions. He does this by articulating a conception of how nature and convention are intimately connected to each other through proper use in the practice of excellence in a way that foreshadows Aristotle. Repositioning Thucydides in the tradition of classical political thought as a predecessor of Aristotle rather than a follower of the sophists has important implications for both theorists and practitioners of world politics. Well aware of the importance of both nature and convention for the practice of excellence, Thucydides recognizes the importance of power politics as well as institutions in human affairs, and yet endorses neither uncritically. He develops a distinctively normative theory of world politics by placing proper use and moral judgement at the centre of his account. As such, the primary message Thucydides gives, to theorists and practitioners alike, is to deplore human suffering and to struggle towards moderation and practical wisdom in politics, making the best use of tendencies in human nature as well as available institutions to that end.