Peloponnesian War, 431-404 B.C., decisive struggle in ancient Greece between Athens and Sparta. It ruined Athens, at least for a time. The rivalry between Athens' maritime domain and Sparta's land empire was of long standing. Athens under Pericles (from 445 B.C.) had become a bastion of Greek democracy, with a foreign policy of regularly intervening to help local democrats. The Spartans, who favored oligarchies like their own, resented and feared the imperialism and cultural ascendancy of Athens.

The war began after sharp contests between Athens and Corinth over Corcyra (now Kérkira; 433) and Potidaea (432). The first important action was the initial invasion of Attica by a Spartan army in 431. Pericles brought the rural population within the walls, and the Athenian fleet began raids, winning victories off Naupactus (now Návpaktos). Meanwhile a plague (perhaps bubonic) wiped out (430-428) probably a quarter of the population of Athens, and Pericles died. His successor, Cleon, won a great victory at Sphacteria (now Sfaktiriá) and refused a Spartan bid for peace.

The Spartan leader Brasidas now brilliantly surprised Athens with a campaign in NE Greece, taking (424) Athenian cities, including Olynthus and Amphipolis. Fighting went on over these even after an armistice (423) and ended in a decisive Spartan victory at Amphipolis, in which Brasidas and Cleon were both killed (422). The new Athenian leader, Nicias, arranged a peace (421), but his rival Alcibiades persuaded the Athenians to invade powerful Syracuse. In the greatest expeditionary force a Greek city had ever assembled, Alcibiades and Nicias both had (415) commands, but before the attack on Syracuse had begun, Alcibiades was recalled to Athens to face a charge of sacrilege. He fled to Sparta; at his advice the Spartans set up a permanent base at Decelea in Attica and sent a military expert, Gylippus, to Syracuse. The incompetent Nicias lost his chance to surprise Syracuse, and after two years his force was wiped out (413).

Soon Persia was financing a Spartan fleet. Alcibiades sailed it across the Aegean, and there was (412) a general revolt of Athenian dependencies. At Athens the Four Hundred, an oligarchic council, managed (411) a short-lived coup, and Alcibiades, who had quit the Spartans, received (410) an Athenian command. He destroyed the Spartan fleet at Cyzicus (410). The new Spartan admiral, Lysander, built (407) a fleet with Persian aid and won a naval battle off Notium, and Alcibiades was driven from Athens. The Athenians won one more victory at Arginusae, near Lesbos, in 406 and again declined an offer of peace.

The next year Lysander wiped out the Athenian navy (at Aegospotamos, 405) and then besieged Athens, which capitulated in 404. Lysander installed an oligarchic government (the Thirty Tyrants) at Athens, which never regained its former importance. For about 30 years afterward Sparta was the main power in Greece.

(431–404 BC) War fought between Athens and Sparta, the leading city-states of ancient Greece, along with their allies, which included nearly every other Greek city-state. Its principal cause was a fear of Athenian imperialism. The Athenian alliance relied on its strong navy, the Spartan alliance on its strong army. The war fell into two periods, separated by a six-year truce. Fighting broke out in 431, with Pericles commanding the Athenians. In the first 10 years, Archidamus led the Spartans to defeats. Plague struck Athens in 429, killing Pericles and much of the army. In 428 Cleon almost convinced Athens to massacre the rebellious citizens of Mytilene on Lesbos, but Athens rescinded the order. In 421 both states agreed to accept the Peace of Nicias. This lasted six years, until Athens launched its disastrous Sicilian expedition. By 413 Athens's forces were demolished. In 411 an oligarchy briefly took power. When democratic leaders were restored by the navy later that year, they refused Spartan peace offers, and the war continued until 405, when the Athenian navy was destroyed at the Battle of Aegospotami with Persian help. Under blockade, Athens surrendered in 404. Its empire was dismantled, and the Spartans installed the Thirty Tyrants.

Learn more about Peloponnesian War with a free trial on Britannica.com.