The Peloponnesian War's significance extends beyond its immediate military outcomes. Thucydides' account of it, written during and after the war by a participant who was exiled for military failure, is the most sophisticated piece of historical analysis from the ancient world and a founding text of international relations theory.

Thucydides' key argument — that the war was caused by the growth of Athenian power and the fear this inspired in Sparta — is what we now call a structural realist argument. States in an anarchic international system respond to power concentrations with countervailing coalitions; the individual choices of leaders matter less than the structural pressures they operate under. This argument has been applied to modern great power conflicts with such frequency that 'the Thucydides trap' is now standard political science terminology.

The war also raises questions about democratic decision-making that retain full force. The Sicilian Expedition (415-413 BC) — the most catastrophic strategic decision Athens ever made — was democratically approved, driven by popular enthusiasm and the rhetoric of ambitious politicians. Thucydides' account is a systematic argument about the mechanisms by which democracies make bad decisions: the power of demagogic rhetoric, the tendency to escalate rather than accept partial success, the neglect of logistical reality.

The war ended Athenian imperial power and began a period of Spartan hegemony that Sparta was temperamentally, institutionally, and economically incapable of maintaining. The real winner of the Peloponnesian War was Macedonia.