The Greek army was about 10,000 strong and already in position when Xerxes and his army arrived. The Greek army was being led by the Spartan king Leonidas. The Persian Army attacked, but the Greeks held fast, and the Persian army suffered heavy losses. Time and time again the Persians attacked, but were turned away by the Greeks.
All seemed favorable for the Greeks, until a Greek traitor by the name of Ephialtes told Xerxes of an alternate route around the pass. This route, known to Leonidas, was only guarded by 1000 volunteering Phocaeans. The Greeks heard about the betrayal and about the Persian army that would soon surround them from the back. They decided to retreat, but Leonidas would stay with 300 other Spartans to hold the pass long enough to make an organized retreat. The last battle at Thermopylae ended with every last Spartan fighting until they were killed. The distraction gave enough time for the rest of the Greek army to retreat into southern Greece. As a memorial to the 300 Spartans that stayed to fight, 3 inscriptions were set up. The first one, in honor of all, read:
Here did four thousand men from Pelops' land
Against three hundred myriads bravely stand.
Go, stranger, and to Lacedaemon tell
That here, obeying her behests, we fell.
The great Megistias' tomb you here may view,
Whom slew the Medes, fresh from Spercheius' fords.
Well the wise seer the coming death foreknew,
Yet scorned he to forsake his Spartan lords.