What caused the people of Athens to join forces with their rival city state Sparta in 480 BC?

In 480 BC, the people of Athens joined forces with Sparta primarily due to the imminent threat posed by the Persian Empire, which was invading Greece. The Persian King Xerxes aimed to conquer the Greek city-states, prompting a need for a united front against the common enemy. Despite their rivalry, Athens and Sparta recognized that their survival depended on collaboration, leading to the formation of the Hellenic League. This alliance culminated in significant battles, notably at Thermopylae and Salamis, where the Greeks successfully resisted Persian advances.