The Achaemenian monarch Cyrus II, the Great (539–530 bce), created the first “World Empire”, which at its greatest extent stretched from Egypt to Central Asia and from Thrace to India. It lasted for some two centuries until the last king, Darius III, was defeated by Alexander the Great. Cyrus ushered in a millennium of the ascendancy of the Iranian peoples, for the Achaemenian kings were Persians, from the area near modern Shiraz. Not only were they ethnically distinct from the previous rulers in western Asia, but they followed a new, essentially monotheistic religion, worshipping the Good Lord, Ahura Mazda. After the Seleucid interregnum they were succeeded, first by a nomadic Iranian dynasty, the Parthians, and then by the Persian Sasanians, who claimed to be descended from them. Both dynasties were to rule for four centuries.

Because Alexander (355–323) died so early, he had no time to consolidate his conquests, and on his death his generals fought before dividing the Achaemenian Empire between them. Alexander’s eastern territories from the Mediterranean to the Oxus fell to Seleucus I (321–281), but he had to fight both the locals and the Greeks settled by Alexander to establish control. Seleucid rule was not to last long: it was challenged as early as 240 by a revolt by the Greek rulers of Bactria, the Greco-Bactrians, and by the recently settled Parthians, who had moved into the former Achaemenian and Seleucid satrapy to the south and east of the Caspian Sea, Parthava, from which they took their name. The ruler who established Parthian power was Mithradates I (c. 171–138), while Mithradates II (124/3–87) confirmed it.