Battle of Thermopylae

The Battle of Thermopylae was fought in 480 BC between the Achaemenid Persian Empire under Xerxes I and an alliance of Greek city-states led by Sparta under Leonidas I. Lasting over the course of three days, it was one of the most prominent battles of both the second Persian invasion of Greece and the wider Greco-Persian Wars.

The engagement at Thermopylae occurred simultaneously with the naval Battle of Artemisium: between July and September 480 BC. The second Persian invasion under Xerxes I was a delayed response to the failure of the first Persian invasion, which had been initiated by Darius I and ended in 490 BC by an Athenian-led Greek victory at the Battle of Marathon. By 480 BC, a decade after the Persian defeat at Marathon, Xerxes had amassed a massive land and naval force, and subsequently set out to conquer all of Greece.

Source: Wikipedia