They were already allies, Sparta having supported theestablishment of the democratic regime in Athens. When the Persianssent the punitive expedition against Athens for interfering in AsiaMinor, Athens called on Sparta for support, however the Spartansarrived to late to join in the battle at Marathon.With the Persian decision to take over all the Greekcity-states, the cities had the option of either agree, as did someof the cities, or fight against it. Sparta and Athens both agreedto join with the city-states which resisted.