What Was the Battle at Thermopylae?
Thermopylae was a pass that the Greeks tried unsuccessfully to defend in battle against the Persians led by Xerxes in 480 B.C. Although the Spartans who led the defense were all killed (and may have known in advance that they would be), their courage provided inspiration to the Greeks, many of whom otherwise might have willingly become part of the Persian Empire (the relevant verb is "medize" from the word Mede). The following year the Greeks did win battles agains the Persians.
Persians Attack the Greeks at Thermopylae:
Xerxes' fleet of Persian ships had sailed along the coastline from northern Greece into the Gulf of Malia on the eastern Aegean Sea towards the mountains at Thermopylae. The Greeks faced the Persian army at a narrow pass there that controlled the only road between Thessaly and Central Greece. The Spartan general and king Leonidas was in charge of the Greek forces that tried to restrain the vast Persian army and keep them from attacking the rear of the Greek navy (under Athenian control). Leonidas may have hoped to block them long enough that Xerxes would have to sail away for food and water.
Ephialtes and Anopaia:
Unfortunately for Leonidas, after a couple of days, a medizing traitor named Ephialtes led the Persians around the pass behind the Greek army. The name of Ephialtes' path behind the pass at Thermopylae (which means "hot gates") is Anopaea (or Anopaia); its exact location is debated.
Leonidas sent away most of the amassed troops.
Greeks Fight the Immortals:
On the third day, Leonidas led his 300 Spartan hoplites (elite troops selected because they had living sons back home) plus the allied Thespians and Thebans against Xerxes and his army of "10,000 Immortals." The Spartan-led forces fought this unstoppable Persian force to their deaths in order to block the pass long enough to keep Xerxes and his army occupied while the rest of the Greek army escaped.
The Aristeia of Dieneces:
According to Cartledge, the Spartan Dieneces was so virtuous (aristeia relates to virtue and to the reward given the most honored soldier, which in this battle was Dieneces) that when he was told that there were so many Persian archers that the sky would grow dark with the flying missiles, his laconic reply was: "So much the better -- we shall fight them in the shade." Spartan boys were trained in night raids, so although this was a show of bravery, there was more to it.
A Bit About Themistocles:
Themistocles was the Athenian in charge of the Athenian naval fleet that in name was under the Spartan Eurybiades. Themistocles had persuaded the Greeks to use the bounty from a newly found vein or silver at its mines at Laurium to build the naval fleet of 200 triremes. When some of the Greek leaders wanted to leave Artemisium before the battle with the Persians, Themistocles bribed and bullied them into staying. Some years later Themistocles was ostracized by the Athenians.
The Corpse of Leonidas:
After Leonidas died, there is a story that the Greeks tried to retrieve the corpse in a gesture worthy of the Myrmidons trying to rescue Patroclus in the Iliad XVII, but to no avail. The Thebans surrendered, the Spartans and Thespians retreated and were shot by Persian archers. The body of Leonidas may have been crucified or beheaded on Xerxes' orders. It was retrieved about 40 years later.
Aftermath:
Persians, whose naval fleet had already suffered seriously from storm damage, then (or simultaneously) attacked the Greek fleet at Artemisium, with both sides suffering heavy losses. According to Peter Green, the Spartan Demaratus (on Xerxes staff) recommended splitting the navy and sending part to Sparta, but the Persian navy had been too heavily damaged to do so -- fortunately for the Greeks.
In September of 480, aided by northern Greeks, the Persians marched on Athens and burned it to the ground, but it had been evacuated.