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Xerxes became king of Persia at the death of his father Darius the Great in 485, at a time when his father was preparing a new expedition against Greece and had to face an uprising in Egypt (Herodotus' Histories, VII, 1-4). According to Herodotus, the transition was peaceful this time. Because he was about to leave for Egypt, Darius, following the law of his country had been requested to name his successor and to choose between the elder of his sons, born from a first wife before he was in power, and the first of his sons born after he became king, from a second wife, Atossa, Cyrus' daughter, who had earlier been successively wed to her brothers Cambyses and Smerdis, and which he had married soon after reaching power in order to confirm his legitimacy. Atossa was said to have much power on Darius and he chosed her son Xerxes for successor.
After quelling the revolt of Egypt, Xerxes finally decided to pursue the project of his father to subdue Greece, but made lengthy preparations for that. Among other things, remembering what had happened to Mardonius' expedition a few years earlier (his fleet had been destroyed by a tempest in 492 while trying to round Mount Athos), he ordered a channel to be opened for his fleet north of Mount Athos in Chalcidice. He also had two boat bridges built over the Hellespont near Abydus for his troop to cross the straits.
The expedition was ready to move in the spring of 480 and Xerxes himself took the lead. Herodotus gives us a colorful description of the Persian army that he evaluates at close to two million men and about twelve hundred ships (Histories, VII, 59-100). Modern historians find these figures irrealistic, if only for logistical reasons, and suppose the army was at most two hundred thousand men and the fleet no more than a thousand ships, but this still makes an impressive body for the time. Xerxes' expedition moved by land and sea through Thracia, the fleet following the army along the coast. It didn't meet resistance until it reached Thessalia, where the Persian army defeated the Spartans and their allies at the pass of Thermopylæ while, on sea, neither the Persian nor the Athenian fleet could win the decision in the battle that took place near Cape Artemisium, along the northern coast of the island of Euboea. Because of Themistocles' decision to evacuate Athens, Xerxes managed to take the city and set fire to the temples of the Acropolis, but his fleet was soon after destroyed by the Athenian fleet of Themistocles at the battle of Salamis (Herodotus' Histories, VIII, 83-96 ; a vivid description of the battle of Salamis may also be found in Æschylus' Persians, 272-510).
After this defeat, Xerxes returned to Asia via the Hellespont, leaving part of his army in Greece under the command of Mardonius. But the following year, after having taken Athens a second time, the Persian army was defeated, in September of 479, at Platæa, near Thebes in Boeotia, in a battle that lasted 13 days, in which Mardonius was killed (Herodotus' Histories, IX, 25-85) while, at about the same time, what remained of the Persian fleet was destroyed by a Greek fleet under the command of the Spartan general Leutychides off Cape Mycale, a promontory of the Ionian coast, north of Miletus, facing the island of Samos (Herodotus' Histories, IX, 90-106). This was not the end of the war between Persia and Greece, but it was the end of the incursions of the Persian army on mainland Greece. And without a fleet, Persia had to abandon control of the sea to Athens.
Xerxes died in 465, assassinated probably upon order by one of his sons, Artaxerxes, who succeeded him.
In the Laws, Plato compares Xerxes to Cambyses in that, as him, he was victim of his education at the court, unlike his father Darius, who was not a son of king (Laws, III, 695c-e). And he goes on to say that it is almost impossible for someone raised in an extremely rich family to become virtuous, and to explain thus why there was no other great king of Persia after Darius. But it is Xerxes who serves as an example to Callicles in the Gorgias to show that the stronger should have a greater share (Gorgias, 483c-e).