This paper makes two main arguments about the relationship between Thucydides, modern realism, and the key conceptual ideas they introduce to situate and explain international politics. First, Thucydides refutes the central claim underlying modern realist scholarship, that the sources of state behavior can be located not in the character of the primary political units but in the decentralized system or structure created by their interaction. Second, however, analyses that discuss Thucydides exclusively with respect to this “third-image” realism do not take into account the most important emendation made to political realism in the last half of the twentieth century, Kenneth Waltz's Theory of International Politics. Waltz reformulates the theory of how anarchic political structures affect the behavior of their constituent units and suggests that the question posed by realism—and to be asked of Thucydides—is not whether states behave according to the Athenian thesis or consistently observe the power-political laws of nature, but whether they suffer “costs” in terms of political autonomy, security, and cultural integrity if they do not. Many scholars are therefore incorrect to assume that demonstrating the importance of non-structural factors in The Peloponnesian War severs the connection between Thucydides and structural realism. Thucydides may in fact be a realist, but not for reasons conventionally assumed.