The Battle of Thermopylae was fought between an alliance of Greek city-states, led by King Leonidas of Sparta, and the Persian Empire of Xerxes I over the course of three days, during the second Persian invasion of Greece. It took place simultaneously with the naval battle at Artemisium, in August or September 480 BC, at the narrow coastal pass of Thermopylae ('The Hot Gates').

Xerxes I,  Xerxes The Great   (born c. 519 bc—died 465, Persepolis), Persian king (486–465 bc), the son and successor of Darius I. He is best known for his massive invasion of Greece from across the Hellespont (480 bc), a campaign marked by the battles of Thermopylae, Salamis, and Plataea. His ultimate defeat spelled the beginning of the decline of the Achaemenid Empire.
Accession to the throne

Darius I, called The Great (558?-486 bc), king of Persia (522-486 bc), son of the Persian noble Hystaspes, and a member of a royal Persian family, the Achaemenids. In 522 bc, on the death of King Cambyses II, a group of Magian priests tried to give the throne to one of their number, the usurper Gaumata; he pretended to be Smerdis (died about 523 bc), the murdered brother of Cambyses II.
Darius, a member of the Achemenides family, raised to the throne of the kingdom of Persia by taking part, in 522, in a plot to assassinate Smerdis, who had assumed the kingship that same year at the death of his brother Cambyses on his way back from Egypt. Both Cambyses and Smerdis were sons of Cyrus the Great, the founder of the Persian Empire.