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. In response, the Athenian politician and general Themistocles proposed that the allied Greeks block the advance of the Persian army at the pass of Thermopylae while simultaneously blocking the Persian navy at the Straits of Artemisium.
Around the start of the invasion, a Greek force of approximately 7,000 men led by Leonidas marched north to block the pass of Thermopylae. The Persian army, estimated by ancient authors to number in the millions, and by modern scholars to be between 120,000 and 300,000 soldiers, arrived at Thermopylae by late August or early September. During two full days of battle, the Greeks blocked the only road by which the massive Persian army could traverse the narrow pass. After the second day, a local resident named Ephialtes revealed to the Persians the existence of a path leading behind the Greek lines. Subsequently, Leonidas, aware that his force was being outflanked by the Persians, dismissed the bulk of the Greek army and remained to guard their retreat along with 300 Spartans and 700 Thespians. It has been reported that others also remained, including up to 900 helots and 400 Thebans. With the exception of the Thebans, most of whom reportedly surrendered, the Greeks fought the Persians to the death in one of history's most famous last stands.