One could logically argue that the Melian Dialogue, which takes place in Book V, Chapter XVII, of The History of the Peloponnesian Wars, by Thucydides, does validate the realist position. The Realist school of thought with respect to international relations posits that nations act in their self-interest, rather than out of any altruistic motives. When Thucydides wrote the Melian Dialogue, he was illustrating the disparate motivations that influence or dictate policy. The overwhelming strength of Athens relative to the tiny Spartan island of Melos presented such a disparity in power that the former was able to dictate terms to the latter without fear of consequences. The Melians, in contrast, can only appeal to altruistic sentiments across the negotiating table that simply don't exist. The following two quotes from the Athenian delegation provide a very good example of this element of the Realist school:
". . .you know as well as we do that right, as the world goes, is only in question between equals in power, while the strong do what they can and the weak suffer what they must."
"You, by giving in, would save yourselves from disaster; we, by not destroying you, would be able to profit from you."
These passages from the Dialogue are considered seminal examples of realism, followed perhaps most significantly by the writings of Niccolo Machiavelli. While both the Melian Dialogue and Machiavelli's The Prince are products of the authors' observations and experiences, neither one in and of itself validates a school of thought. Taken together, they illustrate the prevalence of a certain theory of international relations in practice, which does serve to validate the realist position.