Clearchus (d.400 BC) was a Spartan commander who was forced into exile after seizing power at Byzantium, took part in the revolt of Cyrus the Younger of Persia and was killed in the aftermath of the battle.
Clearchus had some personal connections to Byzantium, where he was a guest-friend before being Spartan commander. In 412 the Spartans decided to send him to the Hellespont if they could take Chios and Lesbos from the Athenians. In the same year a group of eleven commissioners were sent to examine the conduct of Astyoehus, one of the Spartan commanders. This group also had the authority to dispatch Clearchus to the Hellespont if they wanted to.
Clearchus commanded a Spartan contingent at the battle of Cyzicus (410 BC), a major Athenian victory late in the Great Peloponnesian War. His troops took part in the fighting on the mainland, and were defeated by an Athenian force under Thrasybulus. By the end of the battle the entire Spartan fleet had been captured or destroyed. The Athenians then put a garrison at Chrysopolis on the opposite coast of the Bosporus to Byzantium and raided the Byzantine shore.
Clearchus was given fifteen triremes and sent to Byzantium, partly to lift the Athenian blockade and partly to try and cut the Athenian grain route from the Black Sea. The Athenians attempted to intercept his fleet and captured three triremes, but Clearchus and the other twelve ships reached safety. Clearchus became harmost of Byzantium.
In 409 Clearchus was able to fight off an Athenian attack on the city, but supplies were running short. In the winter of 409/8 he left the city to meet up with the Persian satrap Pharnabazus, in the hope that he could get reinforcements or money and use them to carry raids that would force the Athenians to lift the blockade of Byzantium. While he was away some of the Byzantines let the Athenians into the city. Clearchus's men fought hard, but were defeated and the city fell into Athenian hands. Clearchus was put of trial and fined for the loss of the city. As so often happened Spartan rule turned out to be worse than the alternatives, and it was Clearchus's own arrogant behaviour and his decision to seize the limited food supplies for his own men that had triggered the Byzantine revolt.
In 406 Clearchus was present with the Spartan fleet at the battle of Arginusae, the last significant Athenian battle of the Great Peloponnesian War. The Spartan commander Callicratides is reported to have said that Clearchus was the most suited to take command if he was killed in the battle. This was indeed the case, but the battle is best known for the Athenian decision to execute a number of their own victorious admirals for failing to rescue the crews of sunken Athenian warships.
In 403/402, after the final defeat of Athens in the Great Peloponnesian War, Byzantium came under heavy pressure from the Thracians and began to suffer from civil conflicts. The city asked for aid from Sparta, and Clearchus was sent back to the city. He employed a force of mercenaries, and seized control of the city. His rule quickly became tyrannical. He began by killing the chief magistrates of the city at a party and then had thirty of the wealthier citizens strangled and took their wealth. The Spartans tried to get him to leave peacefully, but he refused to leave and eventually they had to send an army under Panthoedas to expel him. Clearchus attempted to defend Selymbria, west of the city, but was defeated in battle at Porus. In the aftermath of the battle he was besieged, and eventually agreed to leave under a truce.
Clearchus then entered the service of Prince Cyrus the Younger, the Persian leader in Asia Minor, and brother of the Emperor Artazerxes II. In 401 Clearchus led a Persian army against the Thracians, but this was only a short-lived campaign.
Cyrus's real aim was to overthrow his brother, and in order to achieve this he gathered a large force of Greek mercenaries. According to Xenophon Clearchus was the only Greek who knew what Cyrus's aim actually was, with the rest believing that they were only taking part in a local operation. As the expedition moved further east the Greeks began to suspect that they were involved in a Persian civil war, and refused to advance any further until Clearchus convinced them that it would be more dangerous to desert the prince.
Clearchus commanded 13,000 Greek troops at the battle of Cunaxa (401 BC). These troops were placed on the Prince's right wing with their flank resting on the River Euphrates. The Greek part of the army was victorious, but Cyrus was killed in the fighting. During the battle Clearchus had refused to obey an order to attack the Persian centre, and Plutarch later blamed him for the defeat.
After the battle the Greeks found themselves isolated in the middle of the Persian Empire, but they were undefeated and the Persians were unwilling to risk a direct attack on them. Instead they pretended to be willing to escort the Greeks back out of the Empire. The satrap Tissaphernes was given command on the Persian side, while Clearchus became the acknowledged commander of the Greeks.
Tissaphernes was eventually able to lure Clearchus into a trap. The two men met and agreed to expel those leaders on either side who were stirring up suspicion between the two sides. Clearchus agreed to bring four generals and twenty more junior leaders to the meeting. Clearchus and the generals were arrested, taken to court and beheaded, while the more junior commanders were killed on the spot. The Greeks were thus forced to appoint a new group of leaders, including the historian Xenophon.