Persian Wars: Battle of Thermopylae

Leonidas at Thermopylae. Public Domain

The Battle of Thermopylae is believed to have been fought in August 480 BC, during the Persian Wars (499 BC-449 BC). Having been turned back at Marathon in 490 BC, Persian forces returned to Greece ten years later to avenge their defeat and conquer the peninsula. Responding, an alliance of Greek city-states, led by Athens and Sparta, assembled a fleet and an army to oppose the invaders. While the former engaged the Persians at Artemisium, the latter assumed a defensive position at the narrow Pass of Thermopylae.

At Thermopylae, the Greeks blocked the pass and beat back Persian assaults for two days. On the third, the Persians were able to flank the Greek position after being a shown a mountain path by a Trachinian traitor named Ephialtes. While the bulk of the Greek army retreated, a force of 300 Spartans led by Leonidas I as well as 400 Thebans and 700 Thespians remained to cover the withdrawal. Attacked by the Persians, the Spartans and Thespians famously fought to the death. Advancing south after their victory, the Persians captured Athens before being defeated at Salamis that September.


Having been turned back by the Greeks in 490 BC at the Battle of Marathon, the Persians elected to begin preparing a larger expedition to subjugate Greece. Initially planned by Emperor Darius I, the mission fell to his son Xerxes when he died in 486. Intended as a full-scale invasion, the task of assembling the necessary troops and supplies consumed several years. Marching from Asia Minor, Xerxes intended to bridge the Hellespont and advance on Greece through Thrace. The army was to be supported by a large fleet which would move along the coast.

As a previous Persian fleet had been wrecked off Mount Athos, Xerxes intended to build a canal across the mountain's isthmus. Learning of Persian intentions, the Greek city-states began making preparations for war. Though possessing a weak army, Athens commenced building a large fleet of triremes under the guidance of Themistocles. In 481, Xerxes demanded tribute from the Greeks in an effort to avoid war. This was refused and the Greeks met that fall to form an alliance of the city-states under the leadership of Athens and Sparta. United, this congress would have the power to dispatch troops to defend the region.

Greek Plans

With war nearing, the Greek congress met again in the spring of 480. In the discussions, the Thessalians recommended establishing a defensive position at the Vale of Tempe to block the Persian's advance. This was vetoed after Alexander I of Macedon informed the group that the position could be flanked through the Sarantoporo Pass. Receiving news that Xerxes had crossed the Hellespont, a second strategy was put forward by Themistocles which called for making at stand at the pass of Thermopylae. A narrow passage, with a cliff on one side and the sea on the other, the pass was the gateway to southern Greece.

Battle of Thermopylae

  • Conflict: Persian Wars (499-449 BC)
  • Dates: 480 BC
  • Armies & Commanders:
  • Persians
  • Xerxes
  • Mardonius
  • approx. 70,000+
  • Greeks
  • Leonidas I
  • Demophilus
  • Themistocles
  • approx. 5,200-11,200 men
  • Casualties:
  • Greeks: approx. 4,000 (Herodotus)
  • Persians: approx. 20,000 (Herodotus)

The Greeks Move

This approach was agreed to as it would negate the Persian's overwhelming numerical superiority and the Greek fleet could provide support in the Straits of Artemisium. In August, word reached the Greeks that the Persian army was nearing. The timing proved problematic for the Spartans as it coincided with the feast of Carneia and the Olympic truce.

Though the de facto leaders of the alliance, the Spartans were prohibited from engaging in military activity during these celebrations. Meeting, the leaders of Sparta decided that the situation was significantly urgent to dispatch troops under one of their kings, Leonidas. Moving north with 300 men from the royal guard, Leonidas gathered additional troops en route to Thermopylae. Arriving, he elected to establish a position at the "middle gate" where the pass was the narrowest and the Phocians had previously built a wall.

Alerted that a mountain trail existed that could flank the position, Leonidas dispatched 1,000 Phocians to guard it. In mid-August, the Persian army was sighted across the Malian Gulf. Sending an emissary to negotiate with the Greeks, Xerxes offered freedom and better land in return for their obedience (Map).

Fighting at the Pass

Refusing this offer, the Greeks were then ordered to lay down their weapons. To this Leonidas reputedly replied, "Come and get them." This reply made battle inevitable, though Xerxes took no action for four days. The constricted topography of Thermopylae was ideal for a defensive stand by the armored Greek hoplites as they could not be flanked and the more lightly armed Persians would be forced into a frontal assault.

On the morning of the fifth day, Xerxes sent troops against Leonidas' position with the goal of capturing the Allied army. Approaching, they had little choice but to attack the Greeks. Fighting in a tight phalanx in front of the Phocian wall, the Greeks inflicted massive losses on the attackers. As the Persians kept coming, Leonidas rotated units through the front to prevent fatigue.

With the failure of the first assaults, Xerxes ordered an attack by his elite Immortals later in the day. Surging forward, they fared no better and were unable to move the Greeks. The next day, believing that the Greeks had been significantly weakened by their exertions, Xerxes attacked again. As on the first day, these efforts were turned back with heavy casualties.

A Traitor Turns the Tide

As a the second day was coming to a close, a Trachinian traitor named Ephialtes arrived in Xerxes' camp and informed the Persian leader about the mountain trail around the pass. Taking advantage of this information, Xerxes ordered Hydarnes to take a large force, including the Immortals, on a flanking march over the trail. At daybreak on the third day, the Phocians guarding the path were stunned to see the advancing Persians. Attempting to make a stand, they formed on a nearby hill but were bypassed by Hydarnes.

Alerted to the betrayal by a Phocian runner, Leonidas called a council of war. While most favored an immediate retreat, Leonidas decided to stay at the pass with his 300 Spartans. They were joined by 400 Thebans and 700 Thespians, while the remainder of the army fell back. While there are many theories regarding Leonidas' choice, including the idea that Spartans never retreated, it was most likely a strategic decision as a rearguard was necessary to prevent the Persian cavalry from running down the retreating army.

As the morning progressed, Xerxes began another frontal assault on the pass. Pushing forward, the Greeks met this attack at a wider point in the pass with the goal of inflicting maximum losses on the enemy. Fighting to the last, the battle saw Leonidas killed and the two sides struggle for his body. Increasingly overwhelmed, the surviving Greeks fell back behind the wall and made a last stand on a small hill. While the Thebans ultimately surrendered, the other Greeks fought to the death. With the elimination of Leonidas' remaining force, the Persians claimed the pass and opened the road into southern Greece.


Casualties for the Battle of Thermopylae are not known with any certainty, but may have been as high as 20,000 for the Persians and around 2,000-4,000 for the Greeks. With the defeat on land, the Greek fleet withdrew south after the Battle of Artemisium. As the Persians advanced south, capturing Athens, the remaining Greek troops began fortifying the Isthmus of Corinth with the fleet in support.

In September, Themistocles succeeded in winning a critical naval victory at the Battle of Salamis which forced the bulk of Persian troops to withdraw back to Asia. The invasion was brought to an end the following year after the Greek victory at the Battle of Plataea. One of the most famous battles of this time period, the story of Thermopylae has been recounted in numerous books and films through the years.

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Your Citation
Hickman, Kennedy. "Persian Wars: Battle of Thermopylae." ThoughtCo, Aug. 26, 2020, Hickman, Kennedy. (2020, August 26). Persian Wars: Battle of Thermopylae. Retrieved from Hickman, Kennedy. "Persian Wars: Battle of Thermopylae." ThoughtCo. (accessed June 1, 2023).