The city that hosted the 481 BC congress forming the Greek alliance against Xerxes was Corinth.
More precisely, the meeting took place at the Isthmus of Corinth under Spartan leadership. Representatives of several Greek city-states gathered to address the threat posed by Xerxes I, who was preparing a massive invasion of Greece. The congress also helped end the conflict between Athens and Aegina so that Greek forces could concentrate on Persia.
The alliance was an unusual achievement because the Greek poleis were normally independent and frequently hostile to one another. Athens and Sparta agreed to cooperate, while Themistocles accepted a Spartan admiral as commander of the allied navy. Some states did not join: Argos stayed out, Crete remained neutral, and Thebes and Thessaly were unwilling to support the alliance at that stage.
The congress developed the strategy that paired a land defense at Thermopylae with a naval blockade at Artemisium. Corinth’s location made it especially valuable: controlling the isthmus could prevent a Persian advance into the Peloponnese if the northern defenses failed.