Peloponnesian War (pĕl´əpənē´zhən), 431–404 BC, 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 BC) 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.
The primary source for the Peloponnesian War (to 411) is Thucydides; Xenophon's Hellenica is an inferior sequel. See also G. E. M. de Ste. Croix, The Origins of the Peloponnesian War (1972); D. Kagan, The Peloponnesian War (2003): V. D. Hanson, A War like No Other (2005).