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.