First Persian invasion of Greece
The first Persian invasion of Greece, during the Greco-Persian Wars, began in 492 BC, and ended with the decisive Athenian victory at the Battle of Marathon in 490 BC. The invasion, consisting of two distinct campaigns, was ordered by the Persian king Darius the Great primarily in order to punish the city-states of Athens and Eretria. These cities had supported the cities of Ionia during their revolt against Persian rule, thus incurring the wrath of Darius. Darius also saw the opportunity to extend his empire into Europe, and to secure its western frontier. The first campaign in 492 BC, led by Mardonius, re-subjugated Thrace and forced Macedon to become a fully subordinate client kingdom part of Persia, after being a vassal to Persia as early as the late 6th century BC, probably in 512 BC.