• "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.

The Battle of Thermopylae was fought in 480 BC.  It was a battle between the Persians and a force of Greeks from various city-states.  The battle was fought because the Persians were trying to invade Greece.

Thermopylae was chosen by the Greeks as a place to do battle because it is the site of a narrow pass between mountains and ocean. A large invasion force must go through this pass to get to the rest of Greece.  Thermopylae was being defended by a large Greek force.  But then someone told the Persians how to get through the mountains and behind the Greeks.  At that point, Leonidas, King of Sparta, knew he was doomed and sent most of his force out of the battle.  He and about 1,000 others stayed behind to delay the Persians as long as possible.  Almost all died.

Thermopylae ended as a Greek loss and the Persians went on to sack and burn cities in Boeotia (part of Greece).  The Greeks did eventually win the war and expel the Persians.  Thermopylae has come down in history as a symbol of patriotism and dedication to a cause because of the sacrifice of the men who fought with Leonidas to delay the Persians even though they knew they were doomed.