Leonidas, king of Sparta from 491 to 480 B.C.; opposed Xerxes, the Persian, who threatened Greece with a large army, and kept him at bay at the Pass of Thermopylæ with 300 Spartans and 5000 auxiliaries till he was betrayed by Ephialtes (q. v.), when he and his 300 threw themselves valiantly on the large host, and perished fighting to the last man.