The nearly four centuries of Sasanian rule which separate the accession of Ardashīr I, in the twenties of the 3rd century A.D., from the fall of Yazdgard III, in the middle of the seventh, have long been viewed as a period of bitter enmity between the Iranian and Roman empires. The rise of the Persian Sasanian dynasty has traditionally been interpreted as the victorious reaction of Oriental resentment against the compromises of the over-hellenized Parthian Arsacids. This interpretation reflects the traditional claim of the Sasanians themselves, that they were the descendants of the ancient kings of Iran, and the notions of the contemporary Romans, who likewise saw the new dynasty as the heir of the Achaemenians, the traditional enemies of the Classical world and hence of themselves. Despite these overt proclamations of hostility, the two empires survived side by side as acknowledged equals, and the ambiguity of this situation has been reflected in modern scholarship as well. In recent times, a number of scholars, while recognizing the endemic antagonism of the two rivals, have gone on to argue for striking similarities and consequently for reciprocal influence in the court ceremonials, art, military, legal, fiscal, and administrative institutions of Byzantium and Iran. Some have even gone so far as to consider seriously the claim occasionally made by mediaeval sources that certain of the later Sasanian rulers: Yazdgard I, Khusrau I Anūshīrvān, and especially Khusrau II Parvēz had secretly been converted to Christianity, and to see an irresistible trend toward monotheism in Iran due to Christian influence but ultimately destined to serve the cause of Islam.