Peloponnesian War, 431–404 B.C., a struggle between Athens and Sparta, and their respective allies. As a result of the war Athens lost its empire and Sparta gained political dominance in Greece. The Peloponnesian War is divided into three parts: the first phase, 431–421; an uneasy peace, 421–414; and the last phase, 414–404. The history of the war written by Thucydides, an exiled Athenian general, is a masterpiece of historical writing.

The basic cause of the Peloponnesian War was the division of Greece into two great rival systems of alliances, neither of which could allow the other to become dominant. Sparta headed the Peloponnesian League, consisting of most of the city-states on the Peloponnesus. Athens ruled an empire of city-states in and along the Aegean Sea. The Spartans were strong militarily on land, the Athenians at sea. Internal strife broke out in most Greek city-states between democratic and aristocratic factions. The democrats allied themselves with Athens, and the aristocrats with Sparta.

Sparta declared war in 431 B.C. and invaded Attica, the peninsula on which Athens lies. The city remained invulnerable, protected by its walls, but the inhabitants were stricken by plague in 430. Pericles, the Athenian leader, died in 429. Athens gained prestige in 425 by capturing 120 Spartans, an unheard-of event since Spartans were supposed to fight to the death. During 424–23 Athens' fortunes declined, as the Spartans captured Athenian-allied cities in Chalcidice and Thrace. In 421, Nicias, an Athenian, negotiated a 50-year peace, but it did not bind Sparta's allies.

In 416, Alcibiades, a new Athenian leader, persuaded the citizens to send an expedition to conquer Sicily. He hoped to cut off Sparta's food supply, and to gain men and resources for an invasion of the Peloponnesus. The large invading force was utterly destroyed in 413, and Athens was never able to regain the offensive.

Meanwhile, in 414, war broke out again in Greece. The main battles were fought at sea, with Sparta receiving aid from Persia. In 405 the Spartan commander, Lysander, ambushed and destroyed the Athenian fleet, anchored at Aegospotami. Athens was then forced by a blockade to surrender. The city walls were torn down in 404, the empire was dissolved, and an oligarchy favorable to Sparta came to power.