Our story starts out with the Persian king Xerxes (apparently husband to the Bible's Esther), deciding to go out and conquer the Hellenes, today known as the Greeks. Persia was the super power of its day and Xerxes was confident he could amass enough men and a large enough fleet to reduce those haughty Greeks. His first challenge was to create a bridge over the Hellespont so that his army could march from what is modern day Turkey. They would then simply walk around the Aegean sea and fight it out. The bridge was made but then a problem happened. From Herodotus:
But when the strait had been bridged over, a great storm came on and dashed together all the work that had been made and broke it up. Then when Xerxes heard it he was exceedingly enraged, and bade them scourge the Hellespont with three hundred strokes of the lash and let down into the sea a pair of fetters. Nay, I have heard further that he sent branders also with them to brand the Hellespont.I love that picture, don't you? Herodotus tells such wonderful stories! While his armies were marching Xerxes came into contact with a man named Artabanos who warned him that he was going into danger. He told him that his large army was a danger to itself as it wouldn't be able to last long without constant supply. He also warned that the large Persian fleet was in danger because it wouldn't have a single friendly harbor it could go to if there were storms. Xerxes went on anyway.
There are two main paths into Greece from the north. One of them was cut off by flooding so the Persians were forced to take a route along the eastern coast. This path leads up to a bottle neck near some hot springs at a place called Hot Gates or Thermopylae. A coalition of the Greek city states decided that this would be the best place to delay Xerxes.
The Spartans took command and their king, Leonidas, led 300 of his men to hold onto the bottleneck. (They had some small support from other cities and from Spartan helots. So the overall number was something like 4000, but let's not step on the legend too much.) These men sealed up the passage and the Persians couldn't easily displace them.
For two days, the Persians tried and tried but they couldn't knock them out. Then, a Greek man named Ephialtes, told the Persians of a path that they could use to outflank the Spartans. They did this on the third day. The Greeks retreated to a hill and were subsequently slaughtered by arrows.
This is one of those important myths in Western history. The idea is that a small force of men voluntarily gave up their lives so that their comrades could organize a defense. I don't know what the counter-factuals suggest would have happened without that delay but in the end it doesn't matter. The story is what's important.