In 499 BCE, the Ionian Greek cities rebelled against the Persian empire. They sent ambassadors to Athens and other Greek cities asking for help. While Sparta refused to go to war against the enormous Persian Empire, Athens sent a small military force to join the rebels, who burned down the Persian provincial capital, Sardis. However, the Persian army eventually defeated the rebels in 494 BCE.
The Persian king Darius I was furious at Athens for helping the rebels. He perceived this as disloyalty on the part of Athenians because a few years earlier the Athenians had asked Persia for an alliance and agreed to offer Persians gifts of earth and water. For Persians, these gifts were a symbolic ceremony indicating that Athens had agreed to become a loyal vassal of the Persian king, but for the Athenians, who at that time were afraid that Sparta might attempt to destroy their democracy, the gifts meant an alliance between equals.
Darius decided to punish the Athenians for their disloyalty and in 490 BCE, he sent a small army commanded by Mardonius into Attica. The Athenians defeated this force at Marathon. In this way, they not only challenged the giant superpower but also demonstrated that Persian holdings in Asia Minor and, in particular, the Ionian cities would not remain safely under Persian control until Persians subjugated European Greece.
This was the conclusion to which Xerxes, the next king of Persia, came. Accordingly, in 484, he started preparing for a massive invasion of Greece, which he launched in 480 BCE. When the Greeks succeeded in defeating the enormous Persian Empire, they changed the course of history for Greece, Western Asia, Europe, and the world.