Thermopylae is the name of a place in Greece where a very important battle happened a long time ago. It was a very narrow passage between some big mountains and the sea, and it was a very important place because it was the only way to get to the city of Athens.
Many years ago, a group of people called the Persians wanted to conquer Greece and take over the city of Athens. The Greeks were very brave and did not want to let this happen, so they sent a small group of soldiers to block the narrow passage at Thermopylae and stop the Persians from getting through.
The Greek soldiers were led by a very famous king named Leonidas, and they were very strong and brave. They had to fight against thousands of Persian soldiers, but they did not give up because they knew how important it was to protect their city.
The battle went on for three days, and even though the Greeks were outnumbered, they fought very hard and did not let the Persians pass. Unfortunately, on the third day, a traitor showed the Persians a secret path around the mountains, and they were able to surround the Greek soldiers.
Despite this, the Greeks fought to the very end, and they were able to kill many Persians before they were all killed. Although they lost the battle, the Greeks were admired for their bravery and what they did at Thermopylae is still remembered today as a great act of courage.