Ionian Revolt
The Greek city-states of Athens and Eretria had supported the unsuccessful Ionian Revolt against the Persian Empire of Darius I in 499–494 BC. The Persian Empire was still relatively young and prone to revolts by its subject peoples. Moreover, Darius was a usurper and had to spend considerable time putting down revolts against his rule. The Ionian Revolt threatened the integrity of his empire, and he thus vowed to punish those involved (especially those not already part of the empire). Darius also saw the opportunity to expand his empire into the fractious world of Ancient Greece.