What happened in the Melian Dialogue described by Thucydides?

In his History of the Peloponnesian War, Thucydides uses the "Melian Dialogue" to illustrate the brutal realpolitik of the Athenian Empire.1

During the expedition against the island of Melos in 416 BCE, the Athenians demanded that the neutral Melians join their confederation. In the dialogue, the Athenians offered no moral justification for their demand, asserting only their superior power and warning the Melians against relying on hope, portents, or oracles. The debate is framed in absolute terms, focusing on the principle that the weaker must submit to the stronger.21

When the Melians refused to submit, the Athenians besieged the island. Upon the Melians' eventual unconditional surrender, the Athenians massacred all men of military age and sold the women and children into slavery. This event serves as a stark example of the degeneration of the Athenian body politic and highlights Thucydides' diagnostic approach to history, where he explores the recurring, often ruthless, patterns of human nature.1