 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.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 client kingdom of Persia, the latter which had been a Persian vassal as early as the late 6th century BC.