The Battle of Thermopylae is 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 takes place simultaneously with the naval battle at Artemisium, in August or September 480 BCE, at the narrow coastal pass of Thermopylae ('The Hot Gates').

The Persian invasion is a delayed response to the defeat of the first Persian invasion of Greece, which had been ended by the Athenian victory at the Battle of Marathon in 490 BCE.

Xerxes has amassed a huge army and navy, and set out to conquer all of Greece.

The Athenian general Themistocles has 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 marches north to block the pass in the summer of 480 BCE.

The Persian army, alleged by the ancient sources to have numbered over one million but today considered to have been much smaller (various figures are given by scholars ranging between about 100,000 and 300,000), arrives at the pass in late August or early September.

The vastly outnumbered Greeks hold off the Persians for seven days (including three of battle) before the rearguard is annihilated in one of history's most famous last stands.

During two full days of battle, the small force led by King Leonidas I of Sparta blocks the only road by which the massive Persian army can pass.

After the second day of battle, a local resident named Ephialtes betrays the Greeks by revealing a small path that leads behind the Greek lines.

Leonidas, aware that his force is being outflanked, dismisses the bulk of the Greek army and remains to guard the rear with 300 Spartans, 700 Thespians, 400 Thebans and perhaps a few hundred others, most of whom ware killed.After this engagement, the Greek navy, under the command of the Athenian politician Themistocles, at Artemisium receives news of the defeat at Thermopylae.

Since the Greek's strategy requires both Thermopylae and Artemisium to be held, and given their losses, the withdrawal to Salamis is decided.

The Persians overrun Boeotia and then capture the evacuated Athens.

The Greek fleet, seeking a decisive victory over the Persian armada, attacks and defeats the invaders at the Battle of Salamis in late 480 BCE.

Fearful of being trapped in Europe, Xerxes withdraws with much of his army to Asia (losing most to starvation and disease), leaving Mardonius to attempt to complete the conquest of Greece.

The following year, however, sees a Greek army decisively defeat the Persians at the Battle of Plataea, thereby ending the Persian invasion.Both ancient and modern writers have used the Battle of Thermopylae as an example of the power of a patriotic army defending native soil.

The performance of the defenders at the battle of Thermopylae is also used as an example of the advantages of training, equipment, and good use of terrain as force multipliers and has become a symbol of courage against overwhelming odds.