After careful preparations, the Persian king Xerxes decided to attack the Yaunâ (Greeks) in the summer of 480. His commanders had warned him that great risks were involved: in 490, at Marathon, about 10,000 Athenians had defeated 25,000 Persians. Therefore, Xerxes prepared himself well and built a very large army. The Greeks, who had against all odds managed to overcome their perennial struggles and had accepted Spartan leadership, understood that if they wanted to survive the invasion, they first had to annihilate the Persian transport fleet, because without its support, the enemy army would be forced to return. The Greek navy therefore made a stand at Artemisium.
Meanwhile, the Spartans, commanded by their king Leonidas, were to keep the coastal road at Thermopylae (the name, "hot gate", is derived from a sulfurous spring in the neighborhood). By occupying this position, the Spartans and their allies would prevent the Persian army from attacking the Greek navy in the rear. In Antiquity, Thermopylae was more narrow than today; the sea reached as far inland as the modern road (cf. this photo).
To synchronize the attack on Thermopylae with the fight at Artemisium, Xerxes waited four days before he ordered his soldiers to attack the contemptibly small Greek garrison of 4,000 men. He first sent the Median and Elamite contingents, which were easily repelled by the defenders of the narrow road.
A second wave of troops consisted of the ten thousand Immortals, who were, according to Herodotus of Halicarnassus (our main source) the royal bodyguard. These elite troops did no better.
The Persian position did not improve during the second day of battle. When Xerxes' soldiers passed through the narrow gap, they were killed by their opponents, who had longer spears and better armor. Against these "men of bronze", the Persians were no match. Many of them fell into the sea and drowned.
If we are to believe Herodotus, it was at this moment that a Greek named Ephialtes told the great king about the possibility to turn the position of the Greek army. There was a mountain path. The story is hardly credible: the Persian scouts were probably perfectly capable of finding the path themselves. Anyhow, during the night, the Immortals, commanded by Hydarnes, made a detour and attacked the Greek contingent that guarded the path.
The exact route of the nightly attack cannot be identified. This is just one of the many tracks in the hinterland of Thermopylae. The fact that a nightly operation was possible, makes it possible to date the battle of Thermopylae to a night with more or less full moon: 17, 18 and 19 September (or one day later).
The Unsolved Riddle
At the beginning of the third day, the Greeks learned that the Persians would soon descend from the mountains and attack their rear. At this point, Herodotus' account becomes confused. In one line (7.219), he says that many Greeks decided to leave and abandoned Leonidas; after this, he says that there were also reports that Leonidas commanded them to go away (7.220). Anyhow, only the Spartans (300 Spartiates and 900 helpers), 700 Thespians, and 400 Thebans remained. It is possible that they also wanted to leave, but that they were trapped when the Immortals arrived. The historian Charles Hignett has famously called the reason why Leonidas stayed "an unsolved riddle", and there's little to add to that conclusion.
Herodotus admits that he is puzzled. Having said that the people abandoned Leonidas and having offered the other interpretation, he wonders why Leonidas might have ordered the soldiers to stay. What follows is introduced with the word gnomê (7.220), which he always uses when he offers a personal opinion: Leonidas decided to stay because an oracle had announced that Sparta would either be destroyed or lose its king. Leonidas preferred the second alternative. It may be true - devotio was not an uncommon military practice - but it smells of propaganda, and the story may well have been invented during the difficult months between the defeat at Thermopylae and the victory at Plataea.
Reportedly, Leonidas ordered his men to go forward against their opponents, who were lashed towards the Spartans by their officers. When Leonidas fell, a bitter struggle over his body broke out. Herodotus tells that the Greeks drove off their enemy four times, and finally succeed in dragging the corpse away. This is too homeric to be true, and again we do not know if this really happened: those who were close to Leonidas, did not live to tell their stories.
Still according to Herodotus, the Thebans, whose support for the cause of Greece was halfhearted, deserted their allies and surrendered. Probably, this has been written with the benefit of hindsight: the Thebans later collaborated with the invader.
It is more probable, however, that the Thebans at Thermopylae were fighting for Greece as well. Only when these soldiers, the most anti-Persian men of Thebes, had been taken captive, their town was prepared to collaborate. With some justification, Herodotus has been accused of "malice" by a later author, Plutarch.
After the death of Leonidas and the end of Theban resistance, the surviving Spartans and Thespians retreated to a small hill, where they were killed by Persian archers. Later generations have always venerated this hill; for example, Philostratus calls it "the loftiest spot" in Greece.note
This part of Herodotus' story is probably correct: after all, the Theban survivors could later have told it to him, and the inhabitants of the nearby villages must have known the hill, which was still covered with arrowheads in the nineteenth century. ( Reportedly, a local blacksmith found many of them and was happy with a large supply of raw material.) Modern archaeologists have also found several arrowhead; similar projectiles have been excavated in Asia and in Greek towns, where they were dedicated to the eternal gods.
After the fall of Thermopylae, the road to Greece was open. Artemisium was evacuated, and it became unavoidable that Thebes would be captured and Athens sacked. It was only during the naval battle of Salamis that Greece's fortunes were restored, although the winter of 480/479 was an uneasy one, and it was only in the summer of 479 that the Persians were decisively defeated.
The story of the three hundred Spartans (and their usually forgotten allies from Thespia and Thebes), as told by Herodotus, has become a "classic". Today, there's a reconstruction of the epitaph of the Spartan soldiers:
Stranger, go tell the Spartans that here we are buried, obedient to their orders.
There are also two modern monuments, one dedicated to Leonidas, one to the Thespians.