Negotiations
Mardonios the new Persian generalissimo declared after Salamis:"The Cypriots, the men of Phoenicia, Cnide and Egypt, only were defeated, not the Persians who could not fight. This state of mind is indicative of the Persians' desire to continue the fight despite the departure of Xerxes I. However Mardonios considers it impossible to continue operations at the approach of the bad season and takes up winter quarters in Thessaly. He took the opportunity to launch intense diplomatic maneuvers with Athens, which he hoped to detach from the rest of his allies. But the embassy sent to the Attic capital, under the direction of a prince of Macedonia, Alexander, was told that "as long as the sun would follow its usual path" the Athenians would not make an alliance with the Persian sovereign. Worried, the Spartans also send an embassy to counter the argument of the Persians. She is received coolly enough by the furious Athenians that one can doubt their determination. They specify that “the fact of being Greek, of sharing the same blood and the same language, of having common sanctuaries and sacrifices as well as similar customs” prohibits them from treason.
Resumption of hostilities
In the spring Mardonios then invaded Attica again, which was once again evacuated by its inhabitants, reoccupied Athens and settled in Boeotia. A coalition of Peloponnesian forces was created in the spring of 479 BC. AD led by Pausanias, regent of Sparta and nephew of Leonidas I. It includes troops from Sparta, probably 10,000 hoplites and 30,000 to 35,000 auxiliaries, plus 8,000 Athenians and a few thousand men from other cities of Greece, such as Corinth, Epidaurus, Megara, Plataea, Troezene, Chalcis, Phliont, Aegina, etc. The Greeks line up a total of about 110,000 soldiers, which is 3 times less than the Persians, but we have already seen that this was not a handicap. However, this constitutes the largest Greek troop ever assembled, even if it is undoubtedly necessary to greatly reduce the actual numbers here.
The Greeks cross the Isthmus of Corinth, arrive near Eleusis in order to pass into Boeotia. Mardonios chooses a location, south of Thebes near Plataea, which should favor his cavalry. Opposite, the Spartans hold the right wing and the Athenians the left wing.
The battle
Pausanias is considered a wise general, able to detect the opponent's weak points, but Mardonios is also considered an excellent tactician and the best Persian general. Moreover, the departure of Xerxes I leaves him with a free hand to fight the battle as he pleases. Each of the two generals wishes, in Plataea, to bring the adversary to launch against his own positions.
Initially Pausanias took position on the foothills of Mount Citherus while Mardonios set up his fortified camp on the other bank of the Asopus river. A Persian cavalry attack on the Greek positions fails but Mardonios has his opponents' supply lines and water points harassed. Pausanias then changes position and settles in the plain where a small massif of hills protects his army from a frontal attack by enemy horsemen. But after ten days the lack of water and food forced him to withdraw, in the middle of the night, to a position closer to his old lines, where refueling was easier. This retreat took place in some confusion and the various Greek units lost contact.
It was then that Mardonios made a fatal error of judgment. He believes that the disorganization among the Greeks allows him to launch an assault, August 27, 479 BC. J.-C., instead of waiting for the quarrels between the various Greek contingents to divide his adversaries. The Persian attack encountered fierce resistance, especially from the Spartans who, although cut off from the rest of the army, occupied an overhanging position protecting them from the opposing cavalry. It is in this fight that Mardonios is killed. The death of their leader, then the assault against their entrenched camp and the arrival of other Greek units which had just defeated the Boeotians, allied with the Persians, led to the defeat of the troops of Xerxes I and their massacre in large numbers. Very few seem to manage to flee and join another body of the Persian army whose leader, Artabazus in conflict with Mardonios, was already turning back towards the Hellespont with about 40,000 men. As for Thebes, which had collaborated with the Persians, it was quickly taken and its leaders were executed. The Greek losses are estimated at around 3000 dead, it is however impossible to assess those of the Persian camp. A huge booty is taken from the camp of Mardonios.