The government of Sassanian Persia had always imagined that the most serious menace to its survival would come from the northeast. Ardashir I had established the Kushan state, which he had subjugated, as a buffer between his empire and the world of the nomads. The vast wall at Gurgan and the defenses of the Caucasus proclaimed Iranian fear of the men of the steppe, and these fortifications may be compared with a looser system of castles along the Roman frontier. The astonishing rise of Palmyra, the predations in the reign of Shapur II, and the achievements of the Lakhm and Ghassan, may have suggested that the Arabs might on occasion torment the two great powers; and the linear defenses, which abutted the Arabian desert to the southwest, announce a prudent fear of the Bedouin and the harmful effect of their razzias. But no one expected that the Arabs would destroy the empire of Iran with such astonishing speed.

The slow decline of Rome’s western empire between the early fifth and late sixth centuries is separated from its final extinction in the east by an interval of nearly a thousand years. The decay and disappearance of the Abbasid and Ottoman empires similarly filled many centuries. A far shorter time, a mere two decades between the victory of Heraclius and the death of Yazdgard III, was required for the humiliation of Iran; and only a little less than a century elapsed before the exiled House of Sasan at last relinquished the hope of expelling the Arab conquerors. The speed and permanence of the failure of the Iranian state are indeed surprising, and seem to require explanation.

Modern writers have claimed that the disintegration of the empire of Iran was hastened by the exhaustion of warfare, by royal decadence and aristocratic divisions, the effects of epidemic disease, corruption and moral decay, and by the centrifugal force of ethnic separatism. One writer has attributed the collapse to nothing more than old age. It is possible to find notices which support, or appear to support, those claims, but none of them, by itself, is wholly satisfactory. We may observe that, over the course of four centuries, the Iranian empire had successively overcome foreign invasion, pestilence, and civil war; but in the seventh century of our era, such disasters assailed the empire all at once. Amidst those calamities, jealousy, or fear, had perhaps instructed Kavad II to murder his brothers, and when disease ended the life of that king, the House of Sasan was nearly extinct. The prestige of the ruling family never recovered, and the aristocrats who served it struggled to support a tottering monarchy.

 But we cannot be persuaded by the theory of a distinct identity amongst Iranian aristocrats of real or imaginary Parthian ancestry, who (as the argument goes) happily betrayed their sovereign and ushered in the Arab invaders. Though the impressions of seals and bullas announce the heritage of certain families whose lineage can be traced to the Arsacid era, these are not evidence of a tribal identity, nor even that of a distinct society. The House of Sasan itself claimed authority neither by virtue of regional identity nor because of Persian ethnicity, and they held dominion over Aryans and non-Aryans alike (Aryan was a term used for all people of Iran region). The earliest inscriptions put up by Sasanian kings were regularly composed in three languages: Middle Persian, Greek, and Parthian. We may fairly infer then that the Parthian and Hellenistic identities lingered on in some form in the early days of Sasanian rule, but soon thereafter Parthian and Greek vanish altogether from inscriptions. Such evidence militates against the claim that a distinct Parthian identity persisted until the Arab conquest of Iran. It is also noteworthy that the last vestige of the Sassanian court in exile is a monument erected by the family of Suren – proof that that clan willingly shared in the exile of the House of Sasan.

 It is true that the Arabs could not have crossed the alluvial plain of Babylon, nor could they have traversed the numerous canals and watercourses, without the help of local landowners. The writer Tabari draws our attention to two such men. Bistam the dihqan of Burs constructed a pontoon bridge for the Arab army and the dihqan Shirzad, who had (as it seemed) joined the Arabian side, is said to have advised his new masters to construct siege engines. Such notices, if they are genuine, may attest the Arabs’ skill at employing the knowledge and talents of the peoples whom they subjugated; and the farmers and landowners of Babylon, suspecting that the invaders from the south were soon to become their permanent masters, may have eagerly thrown off the Sassanian yoke. Yet the example of the emperor Julian’s disastrous invasion of Iran suggests that Iranian policy may have aimed at luring the invaders into a trap, and it is possible to doubt the sincerity of Bistam and Shirzad. But, that the defense of the Iranian state was left to aristocrats and local armies was surely a consequence, and not the cause, of the collapse of their empire. The formation of peace treaties by local rulers and priests, and the subsequent outbreaks of rebellion against the conquerors, were policies required by the absence of a central authority.

The first great victory of the Arabs was the capture of the Persian capital and the principle organs of the Iranian administrative state, and herein we surely behold the most important reason for the swift failure of the empire of Yazdgard III. The analysis of Ibn Khaldun remains convincing: when the Arabs captured Ctesiphon, he wrote, ‘the whole Persian empire perished and the regions which remained in Yazdgard’s possession were of no avail to him. This presents, as Ibn Khaldun argued, an obvious contrast with the other great sedentary power of the ancient world. The eastern Roman state was deprived of its richest provinces, but retained its capital at Constantinople; and for more than three centuries the rugged terrain of Anatolia formed an impassable barrier to the successors of Muhammad. An Iranian capital more remote than Ctesiphon would have been guarded by high mountains and inhospitable deserts, but the natural and artificial defenses which fortified the Babylonian plain yielded swiftly to the Arab onslaught. Though the highlands of Iran offered periodic resistance, the subjugation of the capital was permanent.

Conclusion:

For nearly three hundred years Umayyad and Abbasid caliphates bound together the economic and cultural heartlands of the Roman and Iranian worlds. Upon the ruins of the two great sedentary empires, they established and maintained a vast common market of both trade and ideas. This astonishing success proves that the ambition of the House of Sasan was far from unreasonable or impossible. The edifice was erected by the caliphs, but the ground had been loosened and the foundation dug long before. A Parthian king had vowed that he would restore the ancient limits of the empires of Cyrus and Alexander. Ardashir and Shapur had repeated the same boast and asserted a hereditary right to those lands, and the ephemeral successes of Khusro II seemed to announce the achievement of that ancient ambition. But in the end, the civilized world was united not by the successors to the mythical kings from the Abode of the Aryans (Historical name for Iranians), but by those of Muhammad.

Sources used:

Williams, M., Roman-Sasanian Relations (532 to 545 CE), Unpublished MPhil Thesis, Oxford, 2010.

Local Histories of Khurasan and the Pattern of Arab Settlement, Studia Iranica 27, 1998

Tabari = Ta’rikh al-Tabari: Ta’rikh al-Rasul wa’l-Muluk li-Abu Ja’far Muhamad bin Jarir al-Tabari, 10 vols, Muhammad Al-Fadl Ibrahim, Dar al-Ma’arif bi-Misr, Cairo, 1960–1969; translation: The History of Al-Tabari, Yarshater, E., et al. (eds), 40 vols., SUNY Press, Albany, 1985–1997

Baladhuri, Futuh al-Buldan, de Goeje, M. J. (ed.), Brill, Leiden, 1870; translation: Hitti, P. K., The Origins of the Islamic State, 2 vols., Columbia University Press, New York, 1916

Cyrus Cylinder, Finkel, I. L. (trans.), in Curtis, J., et al., The Cyrus Cylinder and Ancient Persia, British Museum Press, 2013