First, Xerxes ordered five thousand archers to shoot a volley of arrows, but the darts did little damage to the Hellenes, who were protected by bronze shields and helmets. Modern scholars have also calculated that the arrows were fired from at least 100 meters away. After that, Xerxes sent ten thousand Medes and Cissians against the Greeks “with orders to take them prisoners and bring them before him.”
The Greeks fought in front of the Phocian wall, in the narrowest part of the pass, so as to use as few soldiers as possible at a time. Ancient writers say that “the men stood shoulder to shoulder” and that the Greeks were “superior in valour and in the measure of their shields.” This is probably a description of the typical Greek phalanx, in which the men formed a wall of shields from which the long doric spears stretched out.
Such an effective array was able to obstruct the entire passage. The Persians, equipped with less long weapons, were unable to shorten the distance to enter into hand-to-hand combat with the hoplites.
According to Herodotus and Diodorus Siculus, the Great King, Xerxes, after assaying the Greek forces, launched a second assault with the Immortals, an elite force of 10,000 men. However, even the chosen unit failed, for using shorter spears than those of the Greeks, it could not assert numerical superiority. The Spartans, moreover, pretended a few times to retreat (taking care, however, to keep their lines compact), and every time the Medes launched themselves disorderly in pursuit, the Spartans turned and faced them, thus killing untold numbers of Persians. Once again, the Persians had to retreat.
After a few days of fighting, while the Persian king, enraged by his losses, was pondering what to do, the turning point in the battle occurred: a man from Heraclea Trachinia named Ephialtes, driven by a desire for a large reward, informed Xerxes of the existence of a mountain path around Thermopylae and offered to lead the Persian army there. This episode made the character of Ephialtes the archetype of the traitor in Greek culture, and his very name took on such a negative connotation in the Greek language that, with time, meant “nightmare.”
Thus, on the third day, because of this trap, the Persians were able to ambush the Greek contingents and set the glorious example of resistance.
Some Greek commanders opted to retreat, but Leonidas decided to remain in defence of the pass with his Spartiate warriors. A contingent of 700 Thespians led by their general Demophilus also refused to leave with the other Greeks and stayed to fight. Also present were 400 Thebans and probably the Iloti who had accompanied the Spartiates.
When most of the army withdrew, only about 3,000 men remained, not including the losses suffered in the previous days: 300 Spartiates, 700 Thespians, 400 Thebans, the 1,000 Phocians stationed above the pass (and perhaps 900 Ilotians, slaves of the Spartans).
At dawn, Xerxes waited for Hydarnes, the Persian commander, to descend the mountain and then send his forces forward. A Persian army of ten thousand men of light infantry and cavalry charged the front of the phalanx. This time, the Greeks left the wall and went out of the pass to clash with the Persians in the broadest part of the gorge, trying to kill as many as possible; they fought with spears until they were all broken and then switched to short swords, called “xiphoi.”
Herodotus reports that two of Xerxes’ brothers, Abrocome and Hyperantheus, died in this melee. Within the first few minutes of the assault, Leonidas had also perished, and furious fights were engaged for his corpse, which ended when the Greeks succeeded in recovering him.
When the Greeks became aware of the arrival of Hydarnes’ men, the battle changed its aspect: the Greeks hurriedly regained the narrows of the pass, overcame the wall, and went to take up position on the hill. About the surviving defenders, Herodotus writes:
“Here they defended themselves to the last, those who still had swords fought with them, and the others resisted with hands and teeth.”
(Herodotus, Histories, VII, 225)
Having demolished part of the wall, Xerxes ordered the hill to be surrounded, and the attackers fought until the last Greek soldier was dead.
In 1939 archaeologist Spyridōn Marinatos, excavating at Thermopylae, found many Persian bronze arrowheads on Kolonos Hill, identifying it as the site of the last Hellenic resistance. Before that time, the hill was believed to be another, smaller and closer to the wall.