While many Athenians celebrated their victory at Marathon and thought that the Persians had gone home. So while Persia delayed through the 480's, Themistocles and the Athenians began a navy-building project. By 481 BC, Athens had a navy of two hundred ships.

When Xerxes gathered his army at the Hellespont, near the Aegean Sea. There some of the Greeks thought of winning against Themistocles powerful army. Of several hundred Greek cities, about only 33 cities decided to keep serving the Persian army. These states were led by Sparta, Corinth, and Athens so the army still got a lot of food and clean water.

Themistocles understood that the battle would be won or lost at sea. He figured that the Persian army could only win the battle if they were supported by supplies. He also understood that the Aegean Sea was a violent place, with dangerous winds and sudden squalls. While Themistocles kept the Athenian fleet safe in harbor, many of Xerxes’ boats were destroyed at sea.

That time came in a sea battle off the island of Salamis. The Greeks had slow, clumsy boats in comparison with the Persian boats, so they turned their boats into transporters. This means that they filled their boats with military units and when they got close enough they attacked the ship either sinking it or capturing it. It was a brilliant innovation, and the Athenians managed to destroy the major portion of the Persian fleet. However, one Persian general, Mardonius, was still alive. He lived in Greece, but he was met in 479 BC by the largest Greek army history had ever known. Under the leadership of the Spartan king, Pausanias, Mardonius was killed in the battle of Plataea, and his army retreated back to Persia.

Soon after the war ended the Persians and the Athenians became allies. The two armies became allies because they kept fighting each other for no real reason. An ally is a person who is on your side and will not fight against you.