Why did Greek city-states form an alliance against Xerxes’s invasion?

Greek city-states formed an alliance against Xerxes’s invasion primarily because they faced an existential threat: the Persian goal was to turn all of Greece into a Persian province or satrapy.1

To counter this, approximately 30 states that were willing to resist organized a general league in 481 BCE. This alliance was essential for several reasons:21

• Coordinated Defense: The Greeks recognized that individual resistance was insufficient. They needed a unified strategy to manage both land and naval forces effectively.23
• Centralized Command: The states agreed to set aside internal quarrels, such as the conflict between Athens and Aegina, and conferred command by land and sea upon Sparta.21
• Strategic Necessity: The alliance allowed for the mobilization of resources, such as the Athenian fleet led by Themistocles, and sought to incorporate the naval assets of other Greek powers.21

By forming this highly centralized organization, the Greeks were able to transition from isolated resistance to a coordinated defense against the Persian military machine.2