Why did the city-states of Athens and Sparta temporarely unite around 500BC?

Athens and Sparta temporarily united around 500 BC primarily to confront the common threat posed by the Persian Empire. The invasion of Persia, particularly during the Greco-Persian Wars, necessitated a coalition of Greek city-states to defend their autonomy and prevent conquest. This alliance, despite their differing political systems and rivalries, was essential for mobilizing resources and military strength against the formidable Persian forces. Their collaboration exemplified a rare moment of unity among the often-fractious Greek city-states.