Battle of Plataea
The previous year, the Persian invasion force, led by the Xerxes in person, won the Battles of Thermopylae and Artemisium, and conquered Thessaly, Boeotia and Attica. However, at the last-ditch naval Battle of Salamis, the Allied Greek navy had won a surprise victory, and that stopped the conquest of the whole Peloponnesus. Xerxes then returned home with some of his army, leaving his general Mardonius to finish off the Greeks the next year.