The Persian Achaemenid dynasty ruled an empire which, at is peak, stretched from the Indus Valley to the Black Sea and West Asia. Three successive Persian emperors, Cyrus, Cambyses and Darius, had ambitions to expand westwards, and extended their control over western Asia Minor, Thrace and Macedonia, which became a vassal kingdom. From 540 BCE the cities of Asia Minor were ruled by native tyrants, overseen by the Persian ‘satrap’ of Sardis. Darius welcomed Greeks who wanted to serve him, and amply rewarded Greeks who were willing to act as soldiers, sailors and artisans. However, the Greeks were rightly concerned by the territorial ambitions of their mighty eastern neighbour.