The Persians and Greeks fight over land for many years. It was really 50/50, sometimes one side would win, then next time the other. 11 years. Darius dies in 485 BC. RIP EMPIRE.
Wars are expensive. They use up resources, and when you lose, you lose land and people. Also when Darius dies, the leaders afterwards aren't very good. They are weak leaders, and everything falls apart when they take control.