Political theorists have long argued about whether or not Thucydides was a realist (believing that international conflict results due to human nature) or a constructivist (believing that conventions of international relations are socially and historically constructed). The reason for this disagreement is that Thucydides bridges the two theories in his History of the Peloponnesian War.
As John Zumbrunnen, associate professor of political science at the University of Wisconsin, asserts in "Realism, Constructivism and Democracy in the History" (linked below), Thucydides' blend of constructivism and realism is best evidenced in the person of Pericles, the famed Athenian statesmen. Through his speeches, Pericles attempts to describe Athens as it truly is (realism), but also lead Athens toward a higher form of itself, emphasizing its most noble qualities and letting go of its baser ones (such as its aggression).
Zumbrunnen--discussing Friedrich Nietzsche's brief comments on Thucydides--explains how Thucydides uses Pericles' speeches to bridge constructivism and realism:
Speech shapes not only identity but, too, those aspects of the political world—power, interest—that realists take as fixed by either human nature or the structure of the international arena.
Thus, Thucydides believed in fixed values such as justice and power, but he also believed that speech could define and shape those value.