Modern readers are often shocked to learn that the Athenians—citizens of a free city who defeated the Persians when they invaded Greece, built the Parthenon, and staged the tragedies of Aeschylus and Sophocles—also massacred the citizens not of an enemy state but of a neutral power. ...
The drama is riveting. In 431 BC a conflict now called the Peloponnesian War had erupted between two sets of cities, one led by Athens and one by Sparta. It had raged for 15 years when the Athenians demanded the allegiance of the heretofore neutral Melians, whose city traced its origin to Sparta. The Melians balked, and at their request, the leaders of the two sides held a private conference.
The Athenians spoke first. With breathtaking frankness they dismissed considerations of justice as irrelevant. Justice could obtain only between equals. "For ourselves," the Athenians said, "we shall not trouble you with specious pretences … since 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."
The Melians claimed the right to hope that they could resist the Athenians' overwhelming power and that the gods might support them. The Athenians responded with contemptuous clarity: "Of the gods we believe, and of men we know, that by a necessary law of their nature they rule wherever they can." When the Melians refused to submit, the Athenians, helped by local traitors, besieged and captured the city. They executed all adult males, sold the women and children into slavery, and sent out colonists of their own to repopulate the island.
--Anthony Grafton, Slate, on Thucydides' understanding of a Hobbesian world