Some contextual resources for the Melian Dialogue

Though the Melians sent a contingent to the Greek fleet at Salamis, it held aloof from the Delian League, and sought to remain neutral during the Peloponnesian War. But in 415 BC, the Athenians, having attacked the island and compelled the Melians to surrender, slew all the men capable of bearing arms, made slaves of the women and children, and introduced 500 Athenian colonists. Thucydides made this event the occasion of one of the most impressive of the "speeches" in his history. (Melos)

The island of Melos, now called Milos.

From Wikipedia, Thucydides:

Thucydides' Melian dialogue is a lesson to reporters and to those who believe one's leaders are always acting with perfect integrity on the world stage.

Friedrich Nietzsche stated: "in Thucydides, the portrayer of man, that culture of the most impartial knowledge of the world finds its last glorious flower."

Nietzsche again, from Twilight of the Idols:

Courage in the face of reality ultimately distinguishes
such natures as Thucydides and Plato: Plato
is a coward in the face of reality--consequently
he flees into the ideal; Thucydides has
himself under control--consequently he
retains control over things.

A brief overview of the Peloponneisian War is found here, and a glance at the Melian Dialogue is here.