Thucydides. Greek historian. Born at Athens about 471 B.C. Is said to have been descended from Olorus, King of Thrace. At the outbreak of the Peloponnesian War Thucydides received a command, but failed to relieve Amphipolis when besieged by Brasidas, and was banished (423 B.C.). After 20 years of exile, during which he is supposed to have written his "History of the Peloponnesian War," Thucydides returned to Athens about 403. Died about 400 B.C.