The Battle of Thermopylae (Battle of 'The Hot Gates') was fought between an alliance of Greek city-states, led by King Leonidas of Sparta, and the Persian Empire of Xerxes I over the course of three days, during the second Persian invasion of Greece. It took place simultaneously with the naval battle at Artemisium in August or September 480 BC.
After this engagement, the Greek navy, under the command of the Athenian politician Themistocles, at Artemisium received news of the defeat at Thermopylae. Since the Greek's strategy required both Thermopylae and Artemisium to be held, and given their losses, the withdrawal to Salamis was decided. The Persians overran Boeotia and then captured the evacuated Athens. However, seeking a decisive victory over the Persian fleet, the Greek fleet attacked and defeated the invaders at the Battle of Salamis in late 480 BC.
Drawing back into the narrowest part of the pass, and retreating even behind the cross wall, they posted themselves upon a hillock, where they stood all drawn up together in one close body, except only the Thebans. The hillock whereof I speak is at the entrance of the straits, where the stone lion stands which was set up in honour of Leonidas. Here they defended themselves to the last, such as still had swords using them, and the others resisting with their hands and teeth; till the barbarians, who in part had pulled down the wall and attacked them in front, in part had gone round and now encircled them upon every side, overwhelmed and buried the remnant which was left beneath showers of missile weapons.
The Spartans lost all hopes either of conquering or escaping, and looked upon Thermopylae as their burying-place. The king, exhorting his men to take some nourishment, and telling them at the same time, that they should sup together with Pluto, they set up a shout of joy, as if they had been invited to a banquet, and full of ardor advanced with their king to battle. The shock was exceedingly violent and bloody. Leonidas himself was one of the first that fell. The endeavors of the Lacedaemonians to defend his dead body were incredible.
...deserters came in, and brought the news that the Persians were marching round by the hills: it was still night when these men arrived. Last of all, the scouts came running down from the heights, and brought in the same accounts, when the day was just beginning to break. Then the Greeks held a council to consider what they should do, and here opinions were divided: some were strong against quitting their post, while others contended to the contrary. So when the council had broken up, part of the troops departed and went their ways homeward to their several states; part however resolved to remain, and to stand by Leonidas to the last.
The battle lasted three days. The first wave of ten thousand Medes was annihilated, and the second wave of elite “Immortals” too failed to make any significant dent in the defences. On the second day fifty thousand Persian soldiers made another unsuccessful assault on the defending Greek forces. Xerxes I withdrew his forces for the time being do draw other battle plans since his current tactics were clearly inadequate. Luckily for him, a Greek traitor named Ephialtes offered to show the Persians a route around Thermopylae that would outflank the Greeks.
[Xerxes] sent heralds to the opposition force requesting that they deliver up their arms. The answer from Leonidas was as Laconic as the Spartans are famous for, no long winded speeches just a few words that sumed up their intentions.
'Molon labe' (come and take them).
A Spartan; Dieneces, who was told about the great number of Persian soldiers, who with their arrows will conceal the sun, answered: 'Our Trachinian friend brings us excellent tidings. If the Medes darken the sun, we shall have our fight in the shade'.
A Greek force of approximately 7,000 men marched north to block the pass in the summer of 480 BC. The Persian army, alleged by the ancient sources to have numbered over one million but today considered to have been much smaller (various figures are given by scholars ranging between about 100,000 and 300,000), arrived at the pass in late August or early September.
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.
Xerxes had spent years planning his invasion of Greece. It was to be his 'divine punishment' for his father Darius' crushing defeat at Marathon in 490 BC...
Across the Hellespont, the narrow channel of water separating Europe from Asia, he had constructed two bridges, each made of over 300 ships tied together by a network of ropes. On the coast a fleet of some 1,200 ships amassed, while on land over 100,000 (or 1.7 million according to Herodotus' exaggerated account) soldiers made camp at Sardis in Turkey, awaiting their orders.
In 490 B.C. another force under Datis, a Mede, destroyed Eretria and enslaved its inhabitants but was defeated by the Athenians at Marathon. Preparations for a third expedition [to conquer Greece] were delayed by an insurrection in Egypt, and Darius died in 486 B.C. before they were completed.