How did the Greek city-states organize their alliance against Xerxes I?

To resist the invasion of Xerxes I in 480 BCE, approximately 30 Greek city-states formed a centralized and efficient alliance organized by Sparta.1

The alliance was structured through a congress of delegates where each state held one vote, and majority decisions were binding on all members. The organization possessed diplomatic, judicial, and recruiting powers, and the states collectively agreed to cease all internal conflicts to focus on the defense.1

Key aspects of this alliance included:
* Command: The congress conferred supreme command by both land and sea upon Sparta. While the Spartan commander in chief consulted with leaders of national contingents, they retained the authority to make final decisions.1
* Military Coordination: The alliance leveraged the strengths of its members, relying on the Spartans and their allies for land operations and the Athenian fleet, led by Themistocles, for naval defense.1
* Strategy: This structure allowed for a coordinated defense, enabling Sparta to determine the optimal time and place to deploy the alliance's forces against the Persian army.1