Leonidas was heir to the Agiad throne and a full citizen ("homoios") at the time of the Battle of Sepeia against Argos (c. 494 BC). Likewise, he was a full citizen when the Persians sought submission from Sparta and met with vehement rejection in or around 492/491 BC. His elder half-brother, king Cleomenes, had already been deposed on grounds of purported insanity, and had fled into exile when Athens sought assistance against the First Persian invasion of Greece, that ended at Marathon (490 BC).