His interest in the world outside the Peloponnesse may have accounted for some of his reputation for insanity among fellow Spartans who tended to be highly insular, conservative, and suspicious of all things foreign.
Cleisthenes and the Athenian aristocrat Isagoras then fought each other for control of Athens. Cleomenes came with an armed force to support Isagoras, and they forced Cleisthenes and the Alcmaeonidae family to go into exile for a second time. Cleomenes also abolished the Boule, a council set up by Cleisthenes, and occupied the Acropolis. The citizens of Athens objected to this and forced him out of the city. The following year Cleomenes gathered an army, with the aim of setting up Isagoras as tyrant of Athens. This army invaded Attica. The Corinthians in his force refused to attack Athens once they learned of Cleomenes' plan, and the invasion failed.
Sparta then proposed to her allies to mount an expedition to restore Hippias as tyrant of Athens. Given that Sparta had been instrumental in the overthrow of Hippias this change in policy was justified because Sparta had discovered that they had been tricked by the Alcmaeonidae into overthrowing Hippias because of the Delphic oracle had been bribed into asking them to do so. According to W G Forrest, Cleomenes was argued this change of position with his allies. However, the allies, led by Corinth, rejected the proposal in the first act of the Peloponnesian League.
Around 494 BC, Cleomenes invaded and defeated Argos at Sepeia killing about 6000 of its inhabitants in a sacred grove. Argos would remain a bitter enemy of Sparta for decades after this attack. It is not clear why the attack on Argos took place. It may have been the result of Spartas’ concerns over Argos and the city’s pro-Persian tendencies, or due to proximity of Argos to the Spartans and thus being a growing threat to the security of the Spartan state.
When the Persians invaded Greece after putting down the Ionian revolt in 494 BC, many city-states quickly submitted to them fearing a loss of trade. Among these states was Aegina, so in 491 BC, Cleomenes attempted to arrest the major collaborators there. The citizens of Aegina would not cooperate with him and the Eurypontid Spartan king, Demaratus attempted to undermine his efforts. Cleomenes overthrew Demaratus, after first bribing the oracle at Delphi to announce that this was the divine will, and replaced him with Leotychides. The two kings successfully captured the Persian collaborators in Aegina.
While in prison, Cleomenes is found dead with his death being ruled as suicide by self-mutilation. He was succeeded by the elder of his surviving half-brothers Leonidas I, who then married Cleomenes' daughter Gorgo.
The veracity of accounts of Cleomenes' insanity and suicide have been the subject of some speculation among modern historians.