The Battle of Plataea took place in 479 BC and was one of the key encounters of the Greco-Persian Wars. In this battle, the allied armies (Sparta and Athens) clashed with the Persian army under the command of Mardonius.
After their defeat at the Battle of Salamis (480 BC), the Persians largely withdrew from Greek territory. The only large Persian army under Mardonius was tasked with an attack southward, for which the commander had about 50,000 to 60,000 men at his disposal. The allied army at that time had about 30,000 men.
The Greeks took up a defensive position which eliminated the main Persian weapon, namely cavalry. This stalemate suited the Greeks better as reinforcements continued to arrive. The first clash came after a mistake by the Megarians, who got into a vulnerable position, and the Persians showered them with many arrows. They were saved from being utterly crushed by Athenian archers.
The next clash came when Mardonios exposed the weakness of the Greeks in the form of a water supply from a distant spring. He therefore began to attack the supply wagons, driving the commander of Pausanias into a corner. The Greeks were then forced to retreat to the nearby town of Plataea, which offered the possibility of obtaining a much-needed source of water. The move into the city was made at night, but the Spartans and Tegeans were slow to withdraw, and so were spotted by Mardonios in the morning. He called for an attack, believing that the Greek armies had become divided and dispersed.
The Spartans responded to the attack by clashing shields with the Tegeans and forming a single phalanx. This was noticed by the Athenians and they began to return to help along with the Corinthians. Gradually the situation evened out, although the centre of the allies was held by the weakened Corinthians, who were gradually pushed deeper into the territory, dividing the whole army. The Spartans were cut off from the battle and had to withdraw.
A turning point ensued, which is disputed because it is not entirely clear how it occurred. The Persians were still pressing the Greek centre, when suddenly a weakened detachment of Tegeans rushed out and attacked the personal guard of Mardonius. The Spartans, too, began to move and return to the battle. Seeing the attack on their king, the Persian cavalry rode to his aid, but on the way they encountered a slow-marching phalanx of Spartans that stopped them. The Tegeans continued to fight furiously towards Mardonius.
The Persian Mede armour stood no chance against the Greek xiphs, and gradually the Spartans began to push back their opponent. At that point, one of the last Tegean hoplites fought his way to Mardonius and killed him. The commandless Persians began to panic until their army began to retreat. Eventually Artabazos, who did not take part in the battle despite having many soldiers at his disposal, also called for a retreat.
King Xerxes' ground army was thus destroyed in Greece.