Cleomenes I (active ca. 520-490 B.C.) was a brilliant but unstable Spartan king who attempted to extend his country's influence outside the Peloponnesus.

A son of Anaxandridas, Cleomenes first displayed his genius in diplomacy in 519 B.C., when the city of Plataea asked Sparta for help against Thebes. He suggested Plataea ask the assistance of Athens, which accepted and promptly became embroiled with Thebes. Aiming thus to divide and conquer, Cleomenes chose as his next step in central Greece to expel the tyrant Hippias from Athens in 510 and try to bring the city into the Peloponnesian League, of which Sparta held the military command. But Cleomenes, failing to install the pro-Spartan oligarch Isagoras as ruler, was forced to withdraw.

In 508 Cleomenes realized that Athens was an implacable opponent of Spartan power in central Greece. He therefore organized a concerted attack against Athens. In 506 a Boeotian army, led...