" We may express the historical function of the 'Abbasid Caliphate by describing it as a reintegration and resumption of the Achaemenian Empire - a reintegration of a political structure which had been broken up by the impact of an external force and the resumption of a phase of social life which had been interrupted by an alien intrusion. "

― Arnold Joseph Toynbee , A Study of History, Abridgement of Vols 1-6