Over time, the Persian Empire began to weaken. Some rulers became too focused on luxury and rich living instead of strong leadership. This made it harder to control such a large empire.
Persia tried many times to defeat the Greek city-states, especially Athens and Sparta. The Persian kings sent massive armies made up of soldiers from all over the empire. These armies included the famous Immortals, war elephants, and hired soldiers called mercenaries.
Even with their powerful armies and smart planning, Persia could not defeat Athens and Sparta. One famous battle tells the story of 300 Spartan soldiers who bravely fought to slow down the huge Persian army of which legend says was made of a million soldiers. Their courage became legendary. The Greek soldiers fought hard to protect their homes and freedom.
Later, a young general named Alexander the Great attacked Persia. His smaller but fast army defeated the Persians in several battles. After this, the Persian Empire fell, ending one of the greatest empires in history.