The incessant wars with Rome, as well as the recurring internal divisions and succession crises coupled with the economic stranglehold being applied by the Kushans to the east, had critically weakened the cohesion of the already loosely constructed Parthian state. In about 220, a revolt against Parthian rule broke out in Persis, in southern Iran, under the leadership of the Sassanid family, the hereditary Zoroastrian priests of Istakhr (Persepolis). Under Artaxerxes or Ardashir (222–240), Sassanid control was quickly extended eastward to Kerman. Buoyed by his initial success, and the lack of a serious Parthian response, Ardashir next turned to the west. Vologases V (207–223), who ruled the Parthian south including Babylonia, appears to have disappeared from the scene after 223, presumably an early victim of Sassanid expansion. Next on the Sassanid agenda was Media, the very heart of the empire. Artabanus belatedly grasped the danger he faced in the Persian uprising and fought desperately but unsuccessfully to stop Ardashir; he was overthrown in 227. Thus, in about five decisive years, the Parthian state was brought to an end after almost five centuries of existence.
The ascendancy of the Sassanids under Ardashir was challenged by a coalition of forces supported by Rome, which had no interest in seeing a powerful Persian dynasty replace the Parthians with whom it had such long experience. The coalition was nominally led by Chosroes I (217–252) of Armenia, himself an Arsacid who had been placed on the throne by the defeated Parthian king Artabanus. Chosroes had attempted to intervene in the conflict with the Persians on behalf of his Parthian overlord, but arrived too late to be of any help. He now assumed the role of patron of the Arsacids and undertook to lead the struggle against the Persian usurper, organizing