Cleomenes I Cleomenes or Kleomenes ( ; Greek " Κλεομένης ; " died c . 489 BC ) was an Agiad King of Sparta in the late 6th and early 5th centuries BC . During his reign , which started around 520 BC , he pursued an adventurous and at times unscrupulous foreign policy aimed at crushing Argos and extending Sparta ' s influence both inside and outside the Peloponnese . He was a brilliant tactician . It was during his reign that the Peloponnesian League came formally into existence . During his reign , he intervened twice successfully in Athenian affairs but kept Sparta out of the Ionian Revolt . He died in prison in mysterious circumstances , with the Spartan authorities claiming his death was suicide due to insanity . Early life . He was the son of Anaxandrides II ( of the Agiad royal house ) and his second wife ( apparently a daughter of Prinetades ) , and was the half - brother of Dorieus , Leonidas I , and Cleombrotus . Although the three younger half - brothers were the sons of Anaxandrides ' first wife and therefore had a better claim to the throne according to tradition , Cleomenes was the eldest son and succeeded his father around 520 BC . He allowed his half - brother Dorieus to mount expeditions outside the Peloponnese , perhaps as a way of expanding Spartan influence and territories , and perhaps to rid himself of a potential rival . His interest in the world outside the Peloponnese 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 , especially that , according to Herodotus , Cleomenes acquired a taste for wine drunk " Scythian fashion " ( unwatered ) . War against Athens . Around 510 BC the Alcmaeonidae family , who had been exiled from Athens , requested that Sparta help them overthrow Hippias , the son of Pisistratus and tyrant of Athens . The Alcmaeonidae , led by Cleisthenes , bribed the oracle at Delphi to tell the Spartans to assist them , and Cleomenes came to their aid . The first attack on Athens was a failure , but Cleomenes personally led the second attack and besieged Hippias and his supporters on the Acropolis . He was unable to force Hippias to surrender , but the Spartans captured some of Hippias ' relatives and took them hostage until he agreed to give up the city . 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 the Delphic oracle had been bribed into asking them to do so . According to W G Forrest , it was Cleomenes who argued for this change of position with Sparta ' s allies . However , the allies , led by Corinth , rejected the proposal in the first act of the Peloponnesian League . The Ionian Revolt and its Aftermath . In 499 BC , Aristagoras , the tyrant of Miletus , came to Sparta to request help from King Cleomenes with the Ionian Revolt against Persia . Aristagoras nearly persuaded Cleomenes to help , promising an easy conquest of Persia and its riches , but Cleomenes sent him away when he learned about the long distance to the heart of Persia . According to Herodotus , Cleomenes ' s young daughter Gorgo warned Cleomenes not to trust a man who threatened to corrupt him . Around 494 BC , Cleomenes invaded and defeated Argos at Sepeia killing a large number of Argives , Herodotus says 6000 ( probably an exaggeration ) , by burning them to death in a sacred grove of Argus . 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 Sparta ' s 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 493 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 . Exile and death . Around 490 BC Cleomenes was forced to flee Sparta when his plot against his co - king Demaratus was discovered , but the Spartans allowed him to return when he began gathering an army in the surrounding territories . However , according to Herodotus he was by this time considered to be insane . The Spartans put him in prison . By the command of his half - brothers , Leonidas I and Cleombrotus , Cleomenes was placed in chains . While in prison , Cleomenes was 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 .