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.
As on 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 by Thebes, invaded western Attica; the Chalcidians of Euboea invaded northern Attica; and the two Spartan kings, Cleomenes and Demaratus, led the army of the Peloponnesian League into southwestern Attica. When it became known that Cleomenes planned to make Isagoras tyrant of Athens, some Peloponnesians withdrew in protest, and King Demaratus took his force out of the line. The Spartan army disbanded, and Athens defeated Boeotia and Chalcis. Cleomenes proposed in 505 to make Hippias tyrant of Athens but failed. The only rival to Sparta in the Peloponnesus was Argos. Cleomenes led a surprise sea borne attack against it about 495 and won a great victory which disabled Argos for a generation.
Overseas, Cleomenes pursued a cautious policy since Sparta was not a naval power. About 515 he had rejected Samos's plea for help against the Persians. When the Ionians under Aristagoras revolted against the Persians in 499, Cleomenes again refused to lend Sparta's help. But when the Persians threatened to invade Greece in 491, Sparta allied itself with Athens, and Cleomenes went to Sparta's ally Aegina to arrest the leaders of a government which had submitted to Persia. He was rebuffed on the grounds that both Spartan kings were required to make a diplomatic intervention valid. Cleomenes knew that he could not obtain the cooperation of Demaratus, and he therefore plotted to oust him. A potential rival, Leotychidas, disputed the legitimacy of Demaratus, and the case was referred to Delphi, where Cleomenes bribed the priests and obtained the god's verdict against Demaratus. Leotychidas replaced Demaratus and, with Cleomenes, arrested the Aeginatans.
But the bribery became known. Cleomenes fled to Thessaly and then to Arcadia, where he fomented opposition to Sparta. Though Sparta reinstated him late in 491, Cleomenes apparently went insane and committed suicide.
Information on Cleomenes is in W. W. How and J. Wells, A Commentary on Herodotus, vol. 2 (1912), and in G. L. Huxley, Early Sparta (1962).