The Battle of Thermopylae took place in 480 BC (at the same time as the naval battle of Artemision) between the Greeks and Persians during the second Persian invasion of Greece. The Persians had been defeated at Marathon ten years earlier, so they prepared a second campaign, led by Xerxes. The Athenian politician and general Themistocles persuaded the Greeks to close the narrow coastal pass of Thermopylae and Artemisium. The Persian who, according to ancient sources, had an army of one million men and according to current sources, one hundred to three hundred thousand men, arrived at the pass in early September.
After four days of waiting, the Persians attacked, but the Greeks resisted their assaults for two days. On the third day, Ephialtes, the traitor, led the Persians to the Greek’s flank. When he learned of this, the Spartan king Leonidas (of Agiadon) disassembled the allied forces to regroup into a new defense of the Greeks in the South, keeping with him at the Gates only elite and volunteer forces, in addition to the legendary 300 Spartans, 900-1000 neighbouring Lakedaemonians, perhaps together with Helots and 400 Thebians and 700 Thespians led by Dimofilos, son of Diadromos.
The Persians decimated the entire Greek force that stayed on the battlefield. The Greek fleet, upon hearing the news, decided to depart from Artemision and retreat to Salamis, where they later achieved a significant victory. The Persians retreated completely after the battles of Plataea and Mycale.
The battle of Thermopylae remains as one of the most important battles in Greek and world history. Mainly morally it stands as a shining example of self-sacrifice and loyalty and obedience to one’s homeland. The battle showed the advantages of the military training Spartans underwent, better equipment and the smart use of landscape formation.