Cleomenes I (klēŏm`ĭnēz), d. c.489 B.C., king of Sparta after 518 B.C. In accordance with Sparta's policy of helping oligarchies in other states at the expense of the tyrants or the people, Cleomenes joined the Athenians in ousting the tyrant Hippias, but to Cleomenes' dismay Cleisthenes Cleisthenes, fl. 510 B.C., Athenian statesman. He was the head of his family, the Alcmaeonidae , after the exile of Hippias, and with Spartan help had made himself undisputed ruler of Athens by 506 B.C.
..... Click the link for more information. , the principal Athenian aristocrat, sided with the people and took the power (510 B.C.). Twice Cleomenes attacked democratic Athens. The first time he expelled Cleisthenes, who, however, quickly returned to power, thus halting Spartan influence. The second time Corinth checkmated Sparta by refusing to help in an attack that would have disturbed the balance of power. Cleomenes' reputation for ruthlessness is due chiefly to his attack (c.494 B.C.) on Argos, in which he slaughtered 6,000 Argives—an exploit that gave Sparta hegemony in S Greece for many years.

Cleomenes I

(died 491 BC) Spartan king (519–491). An Agiad (descendant from the legendary founders of Sparta), he ruled jointly with Demaratus. In 510 he expelled the tyrant Hippias from Athens, then supported the oligarchic party against the democratic Cleisthenes and refused to help Athens combat Persia. His policies did much to solidify Sparta's position as the leading power in the Peloponnesus. He bribed the oracle at Delphi to depose Demaratus, but he was discovered and fled. Though reinstated, he went insane and committed suicide.