Xerxes I amassed a large army and fleet and marched on Greece,demanding the subjugation of each city-state along the route. Hethrew up a large pontoon bridge across the Dardanelles, made ofdecommissioned ships, allowing his troops to cross the waterobstacle. As his force neared Athens, some Greek cities chose toally themselves with Persia.A blocking force of 7,000 Greek soldiers held the pass atThermopylae to force the Persians to attempt to turn it by sea,waiting to pounce and destroy the Persian sea power which kept theGreek city armies at home defending their cities against seaborneinvasion.When the sea battle was lost to the Persians, the blocking forcewas withdrawn, the Spartan and Thespian contingents remaining tocover their withdrawal, and being killed to a man.Athens had evacuated its people to cities in the Peloponnese andXerxes occupied it as a base.The Greek fleet then re-engaged the Persians in the Straights ofSalamis and sank or captured the many of its ships, the remainderwithdrawing to Mycale in Asia Minor leaving the army without asource of supplies and half had to be sent home. This weakenedforce with its Greek allies was defeated at Plataea the followingyear. At the same time the Greek fleet captured the Persian fleetat Mycale, thus ending the invasion.The war continued on spasmodically to a third phase for the next30 years. Sparta gave up the leadership to Athens as it hadinternal problems at home. Eventually the Persians gave up tryingto enforce peace on the Greeks and made peace, leaving the Greekcity-states to go back to their usual fighting amongstthemselves.