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 among its subject peoples.[11][12] Moreover, Darius was a usurper, and had spent considerable time extinguishing revolts against his rule.[11] The Ionian revolt threatened the integrity of his empire, and Darius thus vowed to punish those involved (especially those not already part of the empire).[13][14] Darius also saw the opportunity to expand his empire into the fractious world of Ancient Greece.[14] A preliminary expedition under Mardonius, in 492 BC, to secure the land approaches to Greece ended with the re-conquest of Thrace and forced Macedon to become a fully subordinate kingdom part of Persia.[15][16] It had previously been a vassal as early as the late 6th century BC, but remained having autonomy and not fully subordinate yet.[16]