The Peloponnesian War (431–404 BC) between Athens and Sparta and their respective allies is arguably the most consequential conflict in Greek history, and Thucydides' account of it is arguably the greatest historical work of antiquity.

Thucydides' methodology was self-consciously modern: he distinguished between immediate causes and underlying causes, he sought eyewitness accounts and evaluated sources critically, he presented speeches as his reconstruction of what the speakers must have said given the circumstances rather than verbatim transcriptions, and he sought the patterns in human affairs that would make his history 'a possession for all time' rather than a performance piece for immediate audiences.

The war's outcome — Spartan victory and the brief imposition of the Thirty Tyrants on Athens — was ultimately temporary: the democratic restoration of 403 BC reestablished Athenian democracy, and Sparta's hegemony proved short-lived. The deeper consequence was the exhaustion of both major Greek powers, which left them vulnerable to Philip II of Macedon's expansion in the 4th century BC.

The Melian Dialogue, in Thucydides' account, is the war's most famous episode: the Athenian ultimatum to the neutral island of Melos to join Athens or be destroyed, and the Melians' appeal to justice and divine protection, answered by the Athenian statement that 'the strong do what they can and the weak suffer what they must.' The subsequent Athenian destruction of Melos and enslavement of its population reads, in Thucydides' narrative, as the moral turning point of the war — hubris preceding nemesis.