How Did Islam Conquer Persia?

The Arab Muslim conquest of Sasanian Persia took about twenty years, from the first raids into southern Iraq in 633 CE to the death of the last Sasanian king of kings, Yazdegerd III, near Merv in 651 CE. The decisive moment was the Battle of al-Qadisiyyah in 636 CE, after which the Sasanian capital of Ctesiphon fell and the Sasanian field army ceased to exist as a coherent force. The conquest was a war, not a single battle, and the Arab armies faced a series of Sasanian counter-attacks before the Iranian plateau was fully in their hands. But the Sasanian state was, by 636, a state in deep structural crisis, and the conquest was a battle against a wounded animal.

The principal English-language treatments are Hugh Kennedy’s The Great Arab Conquests (2007) and Fred Donner’s The Early Islamic Conquests (1981); Parvaneh Pourshariati’s Decline and Fall of the Sasanian Empire (2008) is the indispensable revisionist account of the Sasanian side.

The Sasanian state on the eve

The Sasanian state that the Arab armies faced in 633 was, in territorial terms, the largest Iranian empire since the Achaemenids. It stretched from the Tigris to the Oxus, from the Caspian to the Persian Gulf. It had, within living memory, reached the Bosphorus and taken Jerusalem. It was, in fiscal and administrative terms, a sophisticated state with a cadastrally-based tax system, a professional army, and a network of fire temples, schools, and provincial governors. It was, in substance, a state that was not supposed to fall.

And yet it did, and it did quickly. The reasons are structural. The Sasanian state that the Arabs invaded in 633 was a state that had, in 602–628, fought a long and exhausting war with Byzantium, a war that the Sasanians had effectively won in 621 and then lost decisively to the Byzantine counter-offensive of Heraclius. The war had used up the Sasanian state’s military reserves, weakened its fiscal base, and exposed the limits of its administrative reach. The Sasanian state that emerged from the war in 628 was a state in crisis. James Howard-Johnston’s The Last Great War of Antiquity (2021) is the definitive recent treatment of the war and its consequences.

The crisis was compounded by the overthrow of Khosrow II in 628 and the civil wars that followed. Between 628 and 632, at least eight and possibly as many as twelve different claimants to the Sasanian throne competed for power. The civil wars were fought in the heartland, not on the frontiers, and they did more damage to the Sasanian state than the long war with Byzantium. The Zoroastrian church, the dehqans, the aswaran, 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 Sasanian king Yazdegerd III — a youth, probably between 8 and 21 at his accession — came to the throne in 632, in a state that had used up its reserves and lost its internal coherence. The Arab conquest of the 630s and 640s was the final blow.

The Arab invasion

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 then 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 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. The Sasanian royal treasury, the Sasanian archives, and the legendary Carpet of the Spring of Khosrow were all 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. The Sasanian state, although it would continue to exist in the Iranian heartland for another fifteen years, was effectively destroyed.

The military balance

The Arab army, although smaller than the Sasanian or Byzantine armies, was more mobile, more disciplined, and more ideologically committed than the traditional Near Eastern armies. The Arab army was built around a core of light cavalry and infantry, drawn from the Arabian tribes, and was well suited to the desert and steppe environment of southern Iraq. The standard armament was the lance, the sword, the bow, and a small leather or wicker shield. The Arab army had no cataphracts, no war elephants, and no heavy infantry. Its advantages were mobility, discipline, religious motivation, and a tactical doctrine of feigned retreat and envelopment that the Sasanians had used themselves at Carrhae seven centuries earlier, but which the Sasanian army of the 630s was no longer capable of executing.

The Arab army was also supported by a sophisticated system of logistics and administration. The Arab armies of the conquest were supported by a network of supply depots and by a cadre of administrators who had been trained in the Sasanian and Byzantine traditions — the dehqans and the kuttab of the early caliphate were often Sasanian or Byzantine veterans. The Arab conquest was, in this respect, not a tribal invasion but a military revolution that combined new religious zeal with traditional administrative skill.

The Sasanian counter-attacks

The Sasanian state, although weakened by internal crises, was not passive in the face of the Arab invasion. The Sasanian general Rustam Farrokhzad, who commanded the field army at al-Qadisiyyah, was a capable general, and the Sasanian army was still a formidable force on paper. The Sasanian army was, however, defeated at al-Qadisiyyah, and the fall of Ctesiphon in 637 CE marked the end of effective Sasanian resistance in Mesopotamia.

The Sasanian state continued to exist in the Iranian heartland for several years, and the Sasanian governors of the eastern provinces — Khorasan, Sistan, and the mountain districts of the central plateau — continued to resist the Arab conquest. The Sasanian resistance was, however, weakened by internal divisions and by the military pressure of the Arab armies. 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 Sasanian state was at an end.

What the conquest meant

The Arab conquest of Persia was not the end of Iranian culture, and the Sasanian legacy would shape the Islamic world for centuries. The Arab caliphs adopted Sasanian court ceremonial, Sasanian bureaucratic practices, and Sasanian titulature. The Abbasid revolution of 750 CE drew on Sasanian political traditions to a significant extent, although the thesis that it was a Sasanian restoration is contested. 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 Islamic caliphate and by the later Iranian, Turco-Iranian, and Mongol dynasties.

The question of why the Sasanian state fell so quickly has no single answer. The Sasanian state was weakened by the long war with Byzantium, by the civil wars of the 620s, by the plague of 627–628, and by the loss of several major arsenals. The Sasanian command structure was unstable, and the Sasanian king Yazdegerd III was a child or a youth. The Sasanian army, although still formidable, was not the army of the sixth century. The Arab army, by contrast, was at the height of its early energy, motivated by religious conviction, and led by experienced commanders. The Sasanian tactical doctrine of combined-arms warfare was at least the equal of the Arab, but the Sasanian army of 636 was not capable of executing it.

The deeper answer is structural. The Sasanian state was, throughout its history, a coalition of the king of kings with the great noble families and the Zoroastrian church. The coalition held together when the royal centre was strong; it fell apart when the royal centre was weak. By 636 the royal centre had been weak for eight years, and the coalition was no longer functional. The Arab conquest was the external pressure on a state that had already lost its internal coherence.