Although 300 Spartans traveled to Thermopylae, there were actually two who didn't die in the battle. According to Herodotus, King Leonidas I sent Aristodemus away because he had a disease of the eye; another Spartan soldier went with him, Eurytus, but he ordered his helot (his servant) to lead him back to the battle since he was blind and died there. Aristodemus didn't return even though he could have, so when he returned to Sparta, he was shunned and called a "Trembler." He went on to die in the Battle of Plataea, but didn't manage to regain his honor because he lost control (which was something the Spartans didn't like). The other survivor was Pantites; upon returning to Sparta, he was also seen as a "Trembler" and he hanged himself in shame.
The Battle of Thermopylae, fought between the Persians and the Spartans, was the first battle fought during the Persian invasions between 480 and 479 BC. During this battle 300 Spartans and nearly 6000 other Greeks fought against Xerxes and the Persian forces. During the battle, King Leonidas, the King of the Spartans, decided that he would have his troops fight until they could no longer battle. He knew that he was over matched, but also knew that he had a valiant group of men who would fight for their people.
Though they knew they were greatly outnumbered, the Spartans had a great strategy and longer weapons. During Battle the Spartans pretended to retreat, but as the Persians came forward, they quickly changed course and attacked the Persians in a surprise attack. This strategy was quite helpful in killing the Persians and allowing the battle to go on for longer.
In the end, the Persians had too many soldiers and a Greek soldier who was a traitor was able to help identify a path the Persians could take to trap the Greeks. This allowed Persian soldiers to be on both sides of the Greek troops. The Persians were too strong for the Greeks and the valiant Spartan army fought to their death, defeated in this first of the battles of the Persian-Greek wars.