The Greeks realized that it was only a matter of time before the Persians came back. Darius died in 494 BCE, six years after Marathon. His son, Xerxes, would continue his work. The empire had already expanded as far south, north, and east as possible. The only way to go was west, and conquer Europe. The plan was formulated to invade Greece and Greek Sicily before raiding the rich Italian peninsula.

Xerxes started a large buildup of his army and supplies. He sent slaves to cut a canal through the peninsula at Mount Athos so that his fleet would not meet the same fate as his father’s ships. He had a bridge built across the Hellespont for the army to march across. This was done by lining up boats and connecting them with a bridge. The first attempt was destroyed in a storm, which cost the engineers their lives. The second attempt was successful.

Among the Persians was Demaratus. He had been a king of Sparta until he was exiled on false charges. He had served as an advisor to Xerxes in military matters in Asia Minor, but Xerxes did not seem to believe his advice when it came to the Greeks. Xerxes questioned Demaratus about the Greeks. Xerxes wondered if the Greeks would fight or surrender, considering their small number when compared to the might of Persia. Demaratus informed the King that the Spartans would give battle even if they had only a thousand men to take the field. Xerxes questioned this, asking if the Spartans were such men that they could expect to take on ten men each. Demaratus replied "One-against-one, they are as good as anyone in the world. But when they fight together, they are the best of all. For though they are free men, they are not entirely free. They accept the Law as their master. And they respect this master more than your subjects respect you. Whatever the Law commands, they do. And this command never changes: It forbids them to flee in battle, whatever the number of their foes. He requires them to stand firm – to conquer or die."

The Greeks were well aware that the Persians were building up a massive army. Many of the Greek city-states allied with the Persians to prevent their own destruction, especially those closest to Persia such as Thessaly and Macedonia. Athens got a bit of luck when they discovered a new vein of silver. Instead of dividing the profits with their citizens, Thermistocles convinced the assembly to invest the money in building up the navy. They were able to build and man two hundred additional triremes with the money.

A combined Greek army marched north to try to head off the Persian army in Thessaly at the Pass of Tempe, but they determined that the plain was too wide and it would be too difficult to defend against the Persian cavalry and superior numbers. They decided to pull their armies back.

Representatives from all of the Greek city-states that had not allied with Persia met at Corinth to determine the strategy. The city-states from Peloponnesia, including Sparta, wanted to form a defensive line at the isthmus near Corinth. The city-states east and north of this line wanted a defensive line further north. Thermistocles argued that if Athens fell then the Persians would use their navy to go around the defensive line. He argued that an army at Thermopylae would bottle up the Persians and eliminate the effectiveness of their numbers. Thermopylae was at a narrow stretch of land only 50 feet wide from the cliffs to the sea. Thermopylae took its name from the hot springs there that tourists would come to visit. The narrow pass would not be wide enough for the massive Persian army to out flank them, and it would prevent the use of the Persian Calvary. The Greek navy would protect the army’s flank from the Persian navy. Thermistocles even went to the step of putting the command of the army and navy under Spartan command if King Leonidas would lead the combined army.

Leonidas went back to Sparta to ask for dispensation for the Spartan army to miss the approaching religious holiday, Carneia. The Ephors refused the dispensation. They did not agree that the line should be so far north, but favored a defensive line at Corinth. Furthermore, they received an oracle from Delphi that either Sparta would mourn the loss of a King, or find their city sacked. Under Spartan Law, King Leonidas was allowed to march with his Royal Bodyguard of 300 soldiers without needing dispensation. He planned to march out with his 300 and meet up with allies. They would block the pass until the holiday was over and the rest of the Spartan army would meet them. He realized that it was essentially a suicide mission. The 300 were chosen from men who already had a male heir so that no family lines would die out. As he started the march his wife, Gorgo, met him and asked what she should do. He told her "To marry good men and bear good children."

The Spartans met up with allies along the way to increase their numbers to 7000 soldiers (estimates vary from 4,000 to 8,000). At Thermopylae there was an ancient wall built by the Phoecians to prevent raids from Thessaly, but it was now fallen apart. Leonidas immediately went about rebuilding the wall. He also sent a contingent of local Greeks to protect a goat path that went around the position. He was concerned that if the Persians found out about the track then they would be able to come around behind them and outflank them.

The Persians saw the wall being rebuilt but were not concerned by such a small force. They were more concerned with regrouping their army. The Persian army was so large (estimates vary, but about 200,000-250,000 is most agreed upon) that it took 5 days for the back to catch up with the front. They were literally drinking rivers dry. A spy was sent to see the defenses of the Greeks. The spy could not see past the wall, but was surprised to see the Spartans in front of the wall, combing their hair and doing gymnastics.

An envoy went to the Spartans to warn them to surrender. They explained that there were so many Persian archers that when the fired their arrows blotted out the sun. Leonidas responded "How pleasant then, if we’re going to fight them in the shade." Xerxes waited for five days expecting that the small Greek army would turn in flight as they saw the size of the Persian army opposing them. But the Greeks did not flee.

The first day of battle was mid August 480 BC. It started with the Medes attacking the Greeks in the Pass. The Greeks were able to defeat the Medes either by weapon, or by pushing them off the cliff into the sea. When there seemed to be a stalemate the Spartans would start to run back as if fleeing in fear. The enemy would run after them only to find the Spartans wheeling around and slaughtering more. At the back of the Persian line the commanders were whipping their men to storm forward. The Mede line grew thin and Xerxes sent in a second army, the Cissians, who did not fare any better than the Medes. Three times Xerxes is said to have jumped to his feet for concern over his army. As the first day ended the Greeks were still in the pass and many Persians were dead.

On the second day Xerxes sent another envoy to the Spartans. He told them that Xerxes had great respect for their courage and ability. If they put down their weapons and march away then Xerxes will let them live, and would place them at the head of the Persian army, as it’s first unit. When the envoy asked what answer he should give Xerxes Leonidas told him "Molon labe"-- come and take them. Xerxes proceeded to send in the 10,000 Immortals, his best troops commanded by his brother, Hydarnes. Like the Spartans, they were professional disciplined soldiers. But they did not have the armor and weapons to match the Greeks. Again, the results were the same, at the end of the day the Greeks still held the pass, but Xerxes had lost a brother and many of the Immortals. Now the whole Persian army was demoralized to see the crack Immortals defeated.

A traitor, Ephialtes, told Xerxes about the goat path. Xerxes had the traitor lead the Immortals around the path. He expected that the Immortals would be in position behind the army by noon the next day. Leonidas found out that the track was discovered. He dispatched most of the Greek troops to go back and wait for the next battle. The Thebians refused to leave and were given the position to protect the goat path. The Spartans would not withdraw. By this time many of the 300 Spartans were already injured or dead. Leonidas sent several back as messengers to save their lives. A couple of the older ones sensed what Leonidas was doing and refused to go, saying that they were a soldier and not a messenger. Two of the Spartans had lost their vision due to infection. One insisted on fighting anyway and was led blind into the battle. The other was led back to Sparta and soon regained his vision, but was treated by some as a coward. He proved his valor in the battle of Plataea, but was considered too reckless at that battle. Spartans believed that a man’s valor should keep him solid in the line, neither allowing him to run back as a coward, nor leave the line forward in recklessness. Either action would leave a hole in the line and endanger the other Spartans.

On the third day the remaining Spartans attacked with the aim to do as much damage as they could. Leonidas was killed and there was a fight to retrieve his body. The Spartans finally retrieved his body and retreated to a small hillock nearby. The Immortals found the Thebians unprepared on the path and quickly killed them. They came through the gate to surround the Spartans. Xerxes did not wish to risk further casualties and ordered his archers forward. Volleys of arrows finally killed the Spartans.

After the battle the body of Leonidas was identified. He was decapitated and his head put on a stake. The bodies of the Persian dead were quickly buried to hide the fact that so many were killed by such a small group. In all, about 1,000 - 2,000 Greeks died, while the Persians lost more than 20,000.