What really happened at Thermopylae is somewhat different than the movie 300's portrayal of the events. Though the whole of the Persian force under Xerxes at the time, during the second Persian invasion of Greece, was well over one million strong, only a fraction of their entirety happened to be traveling down that part of the coast of southern Greece. The Greeks and their allies, holding the cavernous pass at Thermopylae, which prevented the Persians from moving further south, numbered about four thousand, three hundred of whom were Spartans under King Leonidas. This was the setting of the Battle of Thermopylae, in September of 480 BCE.
Initial Skirmishes
At the pass, after seeing the determination of the Greeks as they remained awaiting his attack, Xerxes sent first a band of Medes and Cissians, a unit of combatants who hailed from regions under Persian control, to fight the Greeks. A Persian detachment under Hydarnes, known as "the immortals," was sent forth to fight for the narrow pass next by Xerxes, as the Medians and Cissians failed (because such a small space was virtually unconquerable regardless of the size of the army backing those in combat.)
The size of the pass at Thermopylae was key to the Greeks maintaining their ground, because fighting was virtually one on one while reinforcements waited before moving forward to replace any fallen comrade. The Persians were not able to gain ground without taking the entire pass. The path the Persians were following narrowed at this spot as the water was to their left, and a cavernous wall, backed by mountains, stood right. Xerxes needed access to continue south, for he knew no alternate route.
Ephialtes, the Traitor
Herodotus includes in his complete narration of the entire Greco-Persian conflict, The Histories, that Ephialtes, seeking compensation from Xerxes, was a man of the area whose treasonous actions caused the Persians to defeat the Greeks posted at the pass. He revealed to Xerxes a path through the mountains which avoided the narrow pass, and brought the Persian detachment under Hydarnes back to their original path on the other side of the pass. In the mountains they dispersed a thousand Phocians, Greek allies, who were guarding this path while the majority of the Greeks present were below holding the narrow cavern.
The Persian Attack and Victory
The Greeks below received word from the Phocians that the Persians were coming by way of this alternate route, of which they had hoped Xerxes would remain oblivious. Some of the Greek force at Thermopylae were sent home by Leonidas, who hoped to fight for Spartan glory, and spare what lives he could, but the Thespians, Thebans, and Spartans remained at post. The fourteen-hundred Greek combatants holding the pass did well, slaying many in the initial Persian unit of ten thousand. Leonidas, the Spartan King fell, as did some of Xerxes’ sons and brothers, but the Persian force was still strong. The remaining Spartans, along with some Thespians who had not yet fallen, drew up on a hill. As the Persians at hand were still several thousand strong and the Greek force mostly depleted, the brave men who had fared much better in the battle than expected, were overwhelmed by the might of the Persian force.
Aftermath
The Persians were victorious at Thermopylae, but the Greek naval victory at Salamis weakened the Persian fleet significantly. Although they still greatly outnumbered the Greeks, Xerxes and the Persians eventually returned to their ground to moderate what territories they already controlled.
