The Battle was fought between an alliance of Greek city-states, led by King Leonidas I of Sparta, and the Achaemenid 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 Athenian politician and general Themistocles had proposed that the allied Greeks block the advance of the Persian army at the pass of Thermopylae, and simultaneously block the Persian navy at the Straits of Artemisium.
A Greek force of approximately 7,000 men marched north to block the pass in the middle of 480 BC. The Persian army, arrived at the pass in late August or early September. During two full days of battle, the small force led by Leonidas blocked the only road by which the massive Persian army could pass. After the second day, a local resident named Ephialtes betrayed the Greeks by revealing a small path used by shepherds. It led the Persians behind the Greek lines. Leonidas, aware that his force was being outflanked, dismissed the bulk of the Greek army and remained to guard their retreat with 300 Spartans and 700 Thespians, fighting to the death.