Founded early in the 5th century BCE against the Persian threat, the Delian League gradually became an Athenian maritime empire. Tribute extracted from allied cities flowed into the marble of the Acropolis, the Athenian fleet, and the city's boundless self-confidence. The land-based Peloponnesian League under Sparta saw this growth as an existential threat. In 431 BCE disputes over Corinth and Megara escalated into open war.

Pericles' strategy was clear: Athens would retreat behind its walls, avoid land battles, and strike the Peloponnesian coasts from the sea. The plan worked in the first year. But in 430 BCE a plague erupted within the crowded walls, killing perhaps a quarter of the population and, a year later, Pericles himself. With his death Athenian politics passed to hard-line demagogues such as Cleon. The punishment of Mytilene and the massacre at Melos — scenes Thucydides recorded with cold precision — show how a democracy behaves when it becomes an empire.

The turning point came with the Sicilian Expedition of 415–413 BCE. The vast fleet Athens sent to take Syracuse was annihilated; thousands of citizens died in the stone quarries of Sicily. Sparta built a fleet of its own with Persian gold and destroyed the Athenian navy at Aegospotami in 405 BCE. The following year Athens surrendered: its walls were pulled down, its fleet disbanded, its democracy briefly suspended.

The war's meaning exceeds its military outcome. Thucydides wrote his account as a case that 'will recur so long as human nature remains the same'; his analysis of power, fear, and hubris remains a foundational text of modern international relations. The Greek city-states never recovered their former political weight; within half a century Macedon would arrive, and after it the Hellenistic world would be born.