I think that a short biography of the leaders at the Battle of Thermopylae would be a good introduction to the events before, during and after the epic battle. I will begin with Xerxes the first, King of Kings and absolute ruler of the Persian Empire.
Xerxes I, son of Darius I (the Great) and Atossa daughter of Cyrus the Great, was born in 522 BC, his name is the Hellenized version of the ancient Persian Ksayarsha (or Ksayar) which means "Ruler of men" or simply "King". Before the death of his father and his ascend to the throne (485 BC), he was commander of Babylon for 12 years, where he acquired military and administrative experience. His father proclaimed him his successor, bypassing his older half-brothers, because in Xerxes the two branches of the Achaemenid dynasty met, thus guaranteeing the safe continuation of the dynasty.
Following his father’s death, two major rebellions: in Babylon and Egypt, shook the foundations of his Kingdom and forced him to campaign against the local Satraps and restore the Empire’s unity by force. He plundered the delta of the river Nile and demolished the walls of Babylon, enslaving most of it’s population. From that point on he seized using the title "King of Babylon and Pharaoh of Egypt" (terms his predecessors used) and was referred to by his subjects as "King of Persia and Media".
These victories improved his status and increased the influence of the pro-war fractions within the royal palace. Xerxes brother-in-law Mardonius (or Mardony) was able to convince Xerxes to lead a campaign against the Greeks. His motives were definitely not economic since he already possessed the most rich lands and kingdoms of the known world. It was more a matter of prestige and a display of his absolute power over all nations. It was considered an outrage that such a small nation was defying the will of the Great King.
This was actually one of the legacies his father Darius had left to him. Following the revolt of the Ionian Greeks (Asia Minor) in 493 BC which was encouraged by mainland Greek city-states (Athens, Eretria and Naxos), and the defeat of the Persians at Marathon (490BC) Darius had sworn revenge and had a slave to remind his daily "oh Great King, do not forget the Athenians". Darius was preparing a new campaign against the Greeks before his death. The campaign was delayed by the revolts in Babylon and Egypt. However, Xerxes did not begin to conquer Greece without adequate preparations. To the contrary, he worked for three whole years, from 484 to 481 BC, gathering the finest troops from all over the Empire and planning the supply and maintenance of a vast army which soon (Spring 480 BC) would cross from Asia to Europe. It is important to mention that in the Persian royal court there was a number of prominent Greeks, expelled by their countrymen. Among them, Xerxes most important advisor, was Demaratus, ex King of Sparta, who had fled his homeland a few years earlier. He also formed an alliance with Carthage and agreed on a simultaneous campaign of the Carthagians against the Greek city-states of Southern Italy and Sicily, thus blockading any re-enforcement of mainland Greece from the powerful Greek kingdoms of Italy.
Such was the determination of Xerxes that he ordered his army to build two floating bridges between Europe and Asia so they could cross the Hellispont entering Europe safely. His men also dug a channel through the peninsula of Mount Athos (the third peninsula of Chalkidiki) so his fleet would bypass the stormy weather at the edge of the peninsula (at the same spot his father’s fleet had been destroyed a decade earlier due to a storm).
Xerxes moved unopposed through northern Greece and Thessaly, until Thermopylae, all Greek city-states immediately surrendered at the sight of his endless hordes. The Battle of Thermopylae, though a bitter victory for Xerxes was the turning point of the Greco-Persian wars. Xerxes plundered Athens in September 480 BC, but was defeated in the naval Battle of Salamis by an outnumbered Greek fleet lead by the Athenian Themistokles (on the same day the Greeks of Southern Italy and Sicily crashed the Carthagians and forced them back to Africa). He returned to Asia after the Battle of Salamis and left Mardonius to continue the campaign. A year later in 479 BC, the Persians were annihilated at the battlefield of Platea where an army of over 100,000 Greeks crashed the troops of Mardonius (estimated about 300,000), killing Mardonius himself. On the same day the Greek fleet destroyed its Persian counterpart in the naval Battle of Mycale off the shores of Asia Minor.
These latter battles concluded Xerxes interest over Greece. He spent the rest of his life in more peaceful works, building grand palaces in Sousa and Persepolis. Very little is known about Xerxes final years, it is said that with age he gradually withdrew from power and lost control of the Empire. He and his oldest son and heir to the throne were assassinated in 465 BC by the commander of his personal guard Artabanus. He was succeeded by Artaxerxes I.
Xerxes tomb was carved in the great rock-complex of Naqsh-e Rustam, located 7km west of Persepolis, close the tomb of his father.