The Greeks received information regarding the numbers and desires of the Persians from spies they sent to Sardis, Xerxes headquarters. Having experience gained from previous Persian campaigns, they knew that in order to battle against such super power , they had to form alliances to fight for one common goal, despite the fact they belonged to different races and were organized into city-states. In Autum of 481 B.C. , A Pan-Hellenic meeting was held in Corinth , having 31 cities attending the meeting besides a few that did not. Unanimously Sparta was given leadership of the army and the fleet under the command of King Leonidas, although the general defense plan was commanded by the Athenian Themistocles, who foresaw the battle will be determined on the seas, if only the pass between Central to Southern Greece was guarded.
Fighting in the open land would be fatal as they lacked cavalry units and were highly outnumbered, 3 to 1. Thermopylae was the perfect site. The pass itself was narrow, situated between the sea and the steep cliffs. It had a length of about 9 km and narrows in three places: at the eastern and western ends there was room for only one carriage, while the middle section was 15 m wide. The Greeks would use this to their advantage, because it was so narrow, even with an army as big as the Persians had, only a small number could genuinely fight at any one time. This enables a minimal manpower to hold off enormous armies for long amounts of time, which is exactly how Spartan combat was fought.
The Hellenic Army, reached Thermopylae at the beginning of August. In total, a force of about 7000 to 8000 men was assembled. The numbers were again given by Herodotus: 300 Spartans, 700 Tegeats, 550 Mantineians, 300 from Arcadian Orchomenos, 1,000 Arcadians in general, 600 Corinthians, 460 from Phlian, 150 Mycenaeans, 800 Thespians, 450 Thebans, the complete army from Opuntion Locrain and 1,000 Phocians.
The Pass would prevent the enemy from developing his land forces while the small channel of the Evripou prevented the possibility of the Hellenic Fleet being encircled. In addition, a fortifications had been built by a 1000 Phocians to the east of the central pass of Thermopylae in Anopaia to defend the other entry. By defending the Pass the Greeks would be able to apply a withdrawal strategy and at the same time wear out the enemy and cause him serious casualties and lower his morale.
They were very fit, agile, used to marching with heavy armory, they knew the land and their weapons were heavier and more effective. The hoplite phalanx that the Spartans formed was a compact, moving metallic wall of shields and lances. In comparison to Persian armory which was minimal, soldiers wore bright robes with chain armors underneath, carried a long shield called spara and spears much shorter than the Greeks or if they were archers a caved wooden bow. Inmortals were more equipped , having a bronze armor, helmet a bronze square shield for better grip and a axe.
Thinking that the Greeks were impertinent and reckless, he sent Medes and Cissians with orders to take the Greeks prisoner and to bring them back alive. The resistance they encountered however was tough, and the same result was achieved by the Immortals who were sent into battle led by General Hydarnes. Their casualties were high and they were forced to withdraw during the night. In fact, as Herodotus characteristically reported, Xerxes jumped up three times from his throne as he watched the battle, fearing for his army.
Like the first day, Persias second attack was again unsuccessful. Forcing the Empire to retreat with high losses. Xerxes was perplexed and while pondering what to do, a Greek named Ephialtes, son of Eyrydimos and Malida, presented himself and informed the King that the Lakedaimonian positions were accessible from the Anopaia path. That same night, Hydarnes and the Immortals proceeded along the path, guided by Ephialtes
With the retreat of the Phoebians at the pass of Anopaia, Leonidas dismissed the remaining Greeks, to be left alone standing with his 300 spartans and a few thebians , encircled agains 10,000 inmortals. Defeat was quick thanks to Persian arrows.
Herodotus was the only writer who recorded the events of the battle but this was done nearly a generation later, so reliability may be questioned. Numbers and dates are often regarded as incorrect due to the tendency of exaggeration from Herodotous. According to Herodotus, the main force was to arrive later, after the completion of the Carneia Festival in Sparta and the Olympic Games. However, the festivals did not prevent the Peloponnesian Fleet to sail, which is very contradictory .
Apart from the exaggeration and reliability, both Herodotous and modern historian Tom Holland agree upon the importance of Thermopylae.The sacrifice of these men remains in history forever as the most characteristic expression of Spartan bravery, according to Tom. Leonidas managed to safeguard the honour and fame of Sparta, as well as the withdrawal of his allies, while at the same time delaying significantly the advancement of Xerxes and causing serious losses to his forces while raising the morale of the Greeks, who finally crushed the Persians during the naval battle at Salamina just a few weeks later.
If it wasnt for Leonidas army and all the other city states that fought alongside, there was a major possibility that Greece would have been lost to Persia, and most of its future influence in the world such as art, philosophy , language, architecture and maths would have been erased. Therefore Leonidas time delay against the Empire was crucial for Athens to prepare for the naval battle at Salamis.