The Fall of the Sasanian Empire (224–651 CE)

What did Yazdegerd III actually inherit?

The conventional picture is that the last Sasanian king of kings, who came to the throne in 632 CE at the age (probably) of 8 to 21, was the inheritor of a dying state — exhausted by the long war with Byzantium, weakened by the great plague of 627–628, and broken by the civil wars that followed the overthrow of Khosrow II in 628. There is something to this picture, but it is not the whole story. The Sasanian state that Yazdegerd inherited was a state that had, within living memory, reached the Bosphorus and taken Jerusalem; it was a state that still controlled the Iranian plateau from Khuzistan to Khorasan; and it was a state whose fiscal, military, and administrative systems were, on a number of measures, still functional. The Arab conquest was not a walkover. It took the Arab armies fifteen years and several major engagements to dismantle the Sasanian state, and the Sasanian governor of Khorasan held out, in some form, until the 660s. Yazdegerd did not inherit a corpse. He was the inheritor of a wounded animal.

This article tries to read the fall of the Sasanian state as a structural failure rather than as a single event, and to give proper weight to the long process of internal decay that preceded the Arab conquest.

The state in 602

The Sasanian state that went into the long war with Byzantium in 602 was the strongest state in the Near East. The fiscal reform of Khosrow I had been consolidated; the savārān (heavy cavalry) had been reorganised; the Zoroastrian church had been subordinated to the crown; the great noble families had been brought, more or less, to heel. The Sasanian army, on the eve of the war, was probably the most effective military force in the ancient world. Khosrow II used it, between 602 and 621, to conquer the Byzantine Near East, Egypt, and parts of Anatolia, and to capture Jerusalem. The Sasanian Empire of 621 was, in territorial extent, the largest it had ever been.

The war of 602–628 was, however, also the war that broke the state. The Byzantine counter-offensive of Heraclius, beginning in 622, was a strategic masterstroke: rather than defending the Syrian front, Heraclius struck at the Sasanian heartland, won a series of engagements in the Armenian highlands and Mesopotamia, and forced the Sasanians to withdraw from their conquests. By 628 the Sasanian state had recovered none of the lost territory and had lost much of its army. James Howard-Johnston’s The Last Great War of Antiquity (2021) is the definitive recent treatment, and the strategic picture he draws is unambiguous: the Sasanian state was, by 628, a great power in deep crisis.

The civil wars (628–632)

Khosrow II was overthrown and killed in 628 CE. The succession was disputed. Between 628 and 632, at least eight and possibly as many as twelve different claimants to the Sasanian throne competed for power. The most important were Siroes (Kavad II), Ardashir III, and Shahrbaraz — the general who had captured Jerusalem in 614 and was the de facto commander of the Sasanian army in the west. Shahrbaraz seized the throne briefly in 630 and was murdered; the cycle continued. Parvaneh Pourshariati’s Decline and Fall of the Sasanian Empire (2008) argues, on the strength of the Pahlavi and Arabic sources, that the civil wars were not simply a contest of individuals but a contest between the great noble families (the wuzurgān) for control of the state. The Pourshariati thesis has shifted the consensus on the period; not all scholars accept it in full, but the older picture of a simple struggle for the throne no longer holds.

The civil wars of 628–632 were destructive in a way that the long war with Byzantium had not been. The war with Byzantium had been fought on the frontiers; the civil wars were fought in the heartland. The Zoroastrian church, the dehqans (the lesser nobility of the Iranian plateau), the aswaran (the lesser cavalry), and the peasantry were all drawn into the conflict. The plague of 627–628, which swept through the Near East with devastating effect, compounded the damage; the population losses are not quantifiable, but the literary sources agree that they were severe. The Sasanian state of 632, when Yazdegerd III came to the throne, was a state that had been fighting itself for four years.

The Arab conquest (633–651)

The Arab Muslim conquest of the Sasanian state began in earnest in 633 CE, with a series of raids into southern Iraq. The Arab armies, led by Khalid ibn al-Walid, defeated the Sasanian forces at the Battle of Walaja in 633 CE and at the Battle of Ullais later that year. The Arab armies moved into the heart of Iraq, and in 635 CE they laid siege to the great Sasanian capital of Ctesiphon. The Sasanian response was the Battle of al-Qadisiyyah in 636 CE, in which the Arab army under Sa’d ibn Abi Waqqas defeated the Sasanian field army under the spahbod Rustam Farrokhzad. The battle was decisive, and the fall of Ctesiphon followed in 637 CE.

The fall of Ctesiphon was the most consequential event in the long history of the ancient Near East. Ctesiphon, the great Sasanian capital on the Tigris, had been one of the largest cities of the ancient world, and its fall marked the effective end of Sasanian rule in Mesopotamia. The Sasanian royal treasury, the Sasanian archives, and the legendary Carpet of the Spring of Khosrow were carried off by the Arab armies as booty. The great Sasanian palace at Ctesiphon — the taq-i kisra, whose 30-metre vault still stands in modern Salman Pak — was left to decay. Hugh Kennedy’s The Great Arab Conquests (2007) and Fred Donner’s The Early Islamic Conquests (1981) are the most useful English treatments of the military campaign; Michael Morony’s Iraq after the Muslim Conquest (1984) is the indispensable account of the post-conquest settlement.

The Sasanian king Yazdegerd III — a youth, probably between 8 and 21 at his accession — fled eastward through the Iranian plateau, seeking support from the eastern provinces. The Sasanian court eventually settled in Merv, in the eastern Iranian province of Khorasan, but the Sasanian king was unable to organise effective resistance. The Arab conquest was not a single event but a long process. The Arab armies faced a series of Sasanian counter-attacks, and the conquest of the Iranian plateau was not completed until the 650s. The last Sasanian king, Yazdegerd III, was killed in 651 CE near Merv, allegedly by a local miller who was angry at the king’s demand for money. The story may be a literary construction; the fact is not in doubt.

The eastern resistance

The Sasanian state continued to exist in the eastern provinces for several years after the fall of Ctesiphon. The Sasanian governors of Sistan, Khorasan, Kabul, and other eastern provinces continued to resist the Arab conquest. The Sasanian resistance was, however, weakened by internal divisions, by the lack of a strong central government, and by the military pressure of the Arab armies. The Sasanian governors of the eastern provinces were eventually defeated or killed, and the eastern provinces were incorporated into the Islamic caliphate.

The Sasanian resistance in the eastern provinces was accompanied by a series of revolts — the so-called Khorasani revolts — that continued into the eighth century. The revolts had multiple causes: religious (the Zoroastrian population resented Arab rule), economic (the Sasanian landed nobility lost their estates), and political (the Arab governors of Khorasan were often heavy-handed). The revolts were suppressed by the Arab armies, often with great brutality. The Sasanian nobility, deprived of their lands and titles, gradually merged with the new Islamic ruling class, and the Sasanian state was replaced by the Islamic caliphate. The process took the best part of a century, and the Sasanian legacy survived in the new Arab ruling class of Khorasan and Transoxiana.

The structural reading

Three structural weaknesses, in my reading, explain the fall. The first was the dependence of the Sasanian state on the great noble families. The Sasanian monarchy was, throughout its history, a coalition of the king of kings with the wuzurgān (great noble families) and the Zoroastrian church. The coalition held together when the royal centre was strong — under Shapur I, under Khosrow I, under Khosrow II in his early years. It fell apart when the royal centre was weak, and the post-628 civil wars were the moment when the coalition fell apart for good. The Arab conquest was, in effect, the external pressure on a state that had already lost its internal coherence.

The second was the demographic shock of the plague of 627–628. The plague, which swept through the Near East with devastating effect, killed a substantial proportion of the Sasanian population. The fiscal, military, and administrative systems that depended on a stable population were disrupted. The recovery, such as it was, was incomplete. The Sasanian state that Yazdegerd III inherited was, in demographic terms, a state with a damaged base.

The third was the loss of the Zoroastrian church’s monopoly of legitimacy. The Sasanian monarchy derived its authority from its relationship with the Zoroastrian church and the farr (royal charisma) that the church certified. The Mazdakite movement of the late fifth and early sixth centuries had already weakened this monopoly. The rise of Manichaean, Christian, and Buddhist communities in the late Sasanian period had further eroded it. The Arab conquest was, in ideological terms, a conquest by a movement that claimed a different and, in the circumstances, more compelling source of legitimacy.

The legacy

The Sasanian state, although destroyed, left a substantial legacy. The Sasanian system of provincial administration, the Sasanian tax system, and the Sasanian tradition of royal patronage of the arts were all adopted by the early Islamic caliphate and by the later Iranian, Turco-Iranian, and Mongol dynasties. The Sasanian art of rock relief, silver plate, and silk textiles was imitated in the Islamic, Byzantine, and medieval European worlds. The Sasanian tradition of kingship, in which the shahanshah stood at the intersection of heaven and earth and ruled by divine favour, was so compelling that it was adopted, with modifications, by the caliphs of Islam themselves; the title shahanshah was used by Iranian rulers into the twentieth century.

The Sasanian state was the longest of the three classical Persian empires, and its fall was the most consequential. The Achaemenid Empire fell to the Macedonians; the Parthian Empire fell to the Sasanians; the Sasanian Empire fell to the Arabs. Each fall ended a chapter of Iranian imperial history, and each fall was followed by a period of foreign rule. The Arab conquest was the longest of these periods of foreign rule, and the most transformative. The Sasanian legacy survived in the Islamic caliphate and in the medieval Iranian dynasties, but the Zoroastrian state that had been the centre of the pre-Islamic Iranian world did not survive. The Zoroastrian community itself survived — in Iran, in India (the Parsis, who left in the eighth and ninth centuries), and in the diaspora — but the Zoroastrian state did not.

The question of what would have happened if Yazdegerd III had been a competent adult, or if the civil wars of 628–632 had not happened, is a question that has no good answer. The Arab conquest was not inevitable; the Sasanian state was, however, structurally vulnerable to it. The state that emerged from the long war with Byzantium in 628 was, in my view, a state that had used up its reserves, and the Arab conquest was the test that exposed the weakness.

For further reading

Pourshariati, Parvaneh. Decline and Fall of the Sasanian Empire (2008).
Howard-Johnston, James. The Last Great War of Antiquity (2021).
Kennedy, Hugh. The Great Arab Conquests (2007).
Daryaee, Touraj. Sasanian Persia: The Rise and Fall of an Empire (2009).
Morony, Michael G. Iraq after the Muslim Conquest (1984).
Crone, Patricia. The Nativist Prophets of Early Islamic Iran (2012).