CLEARCHUS, a Spartan general of the 5th century b.c. After serving in the Hellespont and at the battle of Cyzicus, he became harmost of Byzantium; but, during his absence, the town was surrendered, and he was consequently punished by a fine. He continued, however, to be employed in matters of importance; but at length, after being sent into Thrace to protect the Greek colonies, he was recalled by the Ephors. He refused to obey, and made himself master of Byzantium. Being driven thence, he visited the court of Cyrus, for whom he levied a little army of Greek mercenaries, which he led on the famous Expedition of the Ten Thousand. He was the only one of the Greeks who was acquainted with the real intention of Cyrus; and it was not till they had proceeded too far to retire with safety that he made known the object for which they had been collected. He commanded a division of his countrymen in the battle of Cunaxa (401 b.c.) ; and he led them on their difficult return journey till, being treacherously seized by Tissaphernes, he was sent to the court of Artaxerxes, where he was put to death.