There were other civilizations, besides the ancient Greeks, located around the Mediterranean Sea. One was ancient Egypt. Another was one of the most powerful and well organized civilization of the time. It was called the Persian Empire.
The Persian Empire was huge. It started in Turkey and stretched all the way to the ancient Indus River Valley. The Greeks still had outposts in Turkey. The Greeks sent supplies to help some of these city-states in their struggle to remain independent. The Persians did not appreciate that.
The Persians sent warriors to the Greek peninsula to squash the Greeks. They attempted three times to conquer the Greeks at the famous battles of Marathon, Thermopylae, and Salamis. Each time, the Greeks drove them away.
Xerxes, the Persian King, could not believe that the tiny Greeks had defeated his capable warriors. After the second loss, Xerxes got made. Conquering the Greeks was no longer his goal. He wanted them wiped out entirely. To do so, he sent a huge number of Persian ships to the Greek peninsula. He was so sure he was going to win this third battle that he had his servants bring his chair along to the war. Xerxes and some of his friends sat in comfort at the top of a cliff overlooking the Greek harbor, so they could watch the Greeks die.
Instead, Xerxes watched his own men die. The Greeks had small ships. They moved quickly. They tossed burning wood aboard the huge, cumbersome Persian ships. The Persian ships quickly caught fire. The Persians had to abandon ship. Those Persians who made it to shore were met by the terrifying Spartan army. It was a slaughter. Xerxes escaped, but most of the Persians warriors were killed.