I am not sure what you are asking when you request the "important historical facts" that occurred at Thermopylae because I'm not sure which you'd think were important. It's also hard to know if you're asking about before, during, or after the battle. Luckily for you, I am long winded and don't have a problem going over all three briefly : )
Here are some important facts that led up to the battle:
- Thermopylae was one battle in a larger war, the Second Persian Invasion of Greece.
- The Persians had two goals: punishing the Greeks for encouraging revolts in Persia, and spreading their empire.
- Though King Xerxes was at Thermopylae, it was his father that had come up with the idea before dying.
- The Persians tried before to conquer Greece before during the First Persian Invasion of Greece, but failed.
- Athens and Sparta were the two major forces on the "pro-Greek" side, but there were about 70 others. The majority of Greeks sat it out, not wanting to get involved. A few Greek cities, the biggest of which was Argos, sided with the Persians.
- The Persian army was probably between 70,000 and 300,000. The Greeks, together, are harder to number. They had no common army, so each battle was fought by whatever troops could be mustered to that area.
Important facts during the battle:
- Despite the film "300," the Greeks had about 7,000 men at Thermopyale. It's hard to say how many Persians were actually at the site of the battle, but it wouldn't be a stretch to say that the Greeks were outnumbered at least 5 to 1.
- Thermopylae was chosen by the Greeks because it had the sea on one side and cliffs on the other. At points, the path between the two was about as wide as a car.
- The Greeks sent a large navy there to keep Xerxes from sailing around the bottleneck.
- The Greeks formed a phalanx (a square of men) and clogged the tiny pass. This took only a portion of their forces, and they were able to rotate soldiers in and out to prevent exhaustion.
- The first day, Xerxes sent 10,000 men into the Greeks, but the Greeks had stronger armor and better weapons. The 10,000 had no way to dislodge them.
- Next, Xerxes sent 10,000 of his best men, the legendary Immortals, but they fared no better.
- The second day was no better for the Persians, but a Greek traitor sold him information about a pass over the cliffs that could be used to trap the Greeks.
- Leonidas learned he was going to be surrounded, and most of the Greeks either left or were ordered to retreat. The 300 Spartans stayed, along with about 1200 others.
- The Greeks held out, but were ultimately killed by a rain of arrows from the clifftops.
After the Battle:
- The Persians moved on, sacking Greek towns.
- The Greek navy destroyed much of the Persian fleet in the battle of Salamis, and Xerxes was scared of being trapped. Another battle at Platea killed a lot of them, and he began to head back for Persia.
- Most of the Persians died on the way back of starvation and disease.
This represents the "highlight reel" of Thermopylae and its impact. The battle was famous as a "last stand," but aside from killing a lot of Persians it had little impact on the outcome of the war.