What brought Sparta and Athens together as allies before they became enemies?

The Persians tried to control the incessant wars between the Greek city-states which spilt over into the Persian Empire, disrupting its peace and prosperity. Many of the Greek cities resisted this, and an alliance led by Sparta which included Athens repelled a Persian invasion. Athens also provided initial support to Sparta when it faced internal revolt, however this ended when Athens began establishing an empire of its own out of the Greek cities liberated from the Persian control.