After the delaying battle of Thermopyle, in which the 300 Spartans (and 8700 other Greeks) fought and delayed the massive army of Xerxes, and after the Persian navy was destroyed at Salimis, Xerxes departed for Persia and left a sizable Persian army in Greece to finish the conquest of Greece. In the Spring of the next year, 479 BC, the Persian army of 300,000 met the assembled Greeks at Platea. The Greeks, numbering roughly 50,000, arranged their battle line first, and the Persians then placed their strongest forces opposite what they thought were the strongest Greek forces. Thus the Persians were opposite the Spartans, and were eager to prove their valor. Opposite the Athenians, Plataians and Megarians, were placed the Boeottians, Locrians, Thessalians, and Phokians. And so the Persians arranged each of their many allied forces against the many independent Greek states.
Once the battle lines were set, both sides sought the advice of their diviners as to whether they should attack first or defend. Diviners of both sides said they would win if they defended, but not if they attacked, so both sides waited for the other side to attack. They waited for eleven days, staring across the battle field. Each day the Greek forces grew more numerous, as small contingents arrived to participate.
On the twelfth day, the Greeks decided to rearrange their forces, and move the Athenians to a place opposite the Persians. The Persians saw this and moved their Persian forces to be opposite the Spartans again. After night fell the Greeks decided to move back a small distance to take up a new battle like, for the purpose of being closer to a water source. In the morning light the Persian general saw that the Greeks were gone, and assumed they had dispersed. He led the entire Persian forces at a run after them, and came up against their reformed battle lines. The Persians attacked in an unorganized mass, and often in small groups, and being lightly armored, were slaughtered by the phalanx of the Greeks. The Spartans held against the Persians, and they and the Athenians pressed forward. The Persian general was killed, which caused panic among the Persian allies.
When the Persians were put to flight, all of the Persian allies also fled. The Greeks pursued the surviving Persians and their allies, and captured their camp with many treasures. About 3000 Persians of the army of 300,000 survived to return to Persia.