During the second half of the sixth century BCE, Persian expansion encroached on the Greek world. In 499 BCE, Athens sent military aid to Greeks rebelling against the Great King. In retaliation, Great King Darius mounted the first Persian invasion of Greece. In 490 BCE, the Athenians defeated this invasion at the Battle of Marathon. Darius’s son Xerxes then planned to conquer all of Greece with a massive army and a huge fleet. Just thirty Greek states, led by Athens and Sparta, banded together to resist the invaders.