Although it had once been one of the most powerful empires of Late Antiquity, by 632 CE the Sassanid Empire was a shadow of its former self. The Byzantine-Sassanian War of 602-628 CE had ended with the execution of the Sassanid King Khosrow II. This led to the Sassanid Civil War of 628-632 CE, during which ten claimants to the throne rose and fell. By the time that order was restored, the Sassanid Empire was politically, militarily, and economically devastated. At the same time, a plague had ravaged the population and centralized authority had largely given way to local rule. Yet, the ultimate fall of the Sassanid Empire was brought about by invaders from the south in one of the most remarkable conflicts of the Arab Conquest.
The Fall of the Sassanid Empire: Rise of the Rashidun Caliphate
When the Prophet Muhammad died in 632 CE, his companions held a debate to determine who would now lead them. At the time, it was necessary for the survival of the community that they elect a leader who could unite the different parties. Eventually, they settled on Abu Bakr (c.573-634 CE) a prominent companion, close advisor, and father-in-law of Muhammad. This was disputed by Ali ibn Abi Talib (c.600-661 CE), whose claim as leader of the community was as just as strong.
While the matter was eventually settled in Abu Bakr’s favor, this dispute would have greater repercussions later. Abu Bakr took the title “Caliph,” which is usually translated from the original Arabic as “successor,” “steward,” or “deputy.” This was the beginning of the first “caliphate;” an institution or public office that governs a territory in accordance with Islamic Law. Caliphs are usually considered or claim to be the politico-religious successors to the prophet Muhammad and the leaders of the entire Muslim world. The first four caliphs ruled what has become known as the Rashidun Caliphate or “rightly guided”.
Abu Bakr’s first challenge was to deal with the various tribes of the Arabian Peninsula that had rebelled and apostatized following the death of Muhammad. Many of these tribes argued that their loyalty and adherence to Islam had ended now that Muhammad was dead. The new caliph, Abu Bakr, argued that they had joined the Muslim community and he was now its head. This resulted in a series of conflicts known as the Ridda Wars (632-633) during which the recalcitrant tribes were brought into line. Following this conflict, Abu Bakr dispatched raiders into the rich Sassanid lands of Mesopotamia.
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Sassanid Mesopotamia was a very tempting target: many Arab tribes lived in the region, they were unhappy with the heavy Sassanid taxes, there was much booty to be gained, and the conquest of the region removed a threat to the nascent Muslim community. With this in mind, Abu Bakr dispatched an all-volunteer army of 18,000 under his best general Khalid ibn al-Walid (d.642 CE) into the region. Khalid won a series of four consecutive and decisive victories over the Sassanids. This allowed him to capture several cities which brought most of the region under Muslim control.
However, Khalid was then forced to deal with a rebellion in Northern Arabia. This enabled the Sassanids to assemble a large army. Before the four divisions of the Sassanid army could be united, Khalid struck first, dividing his forces and defeating the Sassanids before they could combine their forces. Khalid then began working to clear the Sassanids out of Southern and Western Mesopotamia so that he could isolate the capital city of Ctesiphon.
Before Khalid could complete this campaign, he was recalled by Abu Bakr and dispatched to Syria to confront the Romans. Abu Bakr turned the campaign over to Umar ibn al-Khattab (c. 583-644 CE), another close companion of Muhammad. His immediate task was to stabilize the situation as the departure of Khalid and his forces had allowed the Sassanids to regain control of much of the region. The Sassanid king Yazgerd III (c.624-651 CE) allied himself with the Romans and prepared a vast army, so that the allies could drive the Arabs from both Syria and Mesopotamia. However, the Sassanids and Romans were not able to coordinate their efforts and were decisively defeated at the battles of Yarmouk and Qadisiyyah in 636 CE. Yazgerd III fled to the east, while Ctesiphon fell in 637 CE after a three month siege.
The Sassanids Strike Back
By 638 CE, the Arab Conquest of Mesopotamia was practically complete, with the Suwad, Tigris valley, and Euphrates valley under Muslim control. While the political situation remained chaotic and Persian raiding continued, it appeared that the Zagros mountains would form the border between the Rashidun Caliphate and Sassanid Empire. By this point, Abu Bakr had died and Umar ibn al-Khattab had become Caliph.
While Umar was dealing with a severe drought and famine that struck the Arabian Peninsula in 638 CE, the Sassanids were plotting their counterattack. Led by Hormuzan, one of the seven most important nobles of the Sassanid Empire, the Persians struck deep into Mesopotamia. Hormuzan was defeated in every one of the campaigns he led into Mesopotamia. In 640 CE he was defeated in the battle of Tuster by the forces of Ammar ibn Yasir, Abu Musa, and Nouman ibn Muqarin. After the battle, Hormuzan was captured and sent to Umar in Medina, where he appears to have converted to Islam and served as an advisor during the conquest of Persia.
After the battle of Tuster, the strategically important city of Susa was besieged and captured in early 641 CE. This opened the way into the heart of Persian territory and brought the entire province of Khuzistan under Muslim control. At this point, Umar appears to have sought peace with the Persians, who, though defeated, were still powerful. Persian pride and disdain for the Arabs made peace impossible. Yazgerd III gathered an army said to have numbered some 100,000 from all over the Sassanid Empire, which he concentrated at Nahavand. In response, an Arab army of some 30,000 under Nouman ibn Muqarin was dispatched. Through careful maneuver, skirmishing, and feigned retreats, the Muslims scattered and destroyed the Sassanid army in the resulting battle of Nahavand, dealing the Sassanids a decisive defeat.
After the battle of Nahavand, Yazgerd III fled further to the east while attempting to raise a new army. Meanwhile, Umar prepared a three-pronged invasion of Persia that would strike the North, South, and Center of the empire. The first target was the central province of Isfahan as it served as an important conduit for supplies and communication. Additionally, if Isfahan was captured, the other parts of the Sassanid empire would be isolated from each other. In preparation for the invasion, Umar took great care in selecting the commanders at every level of the command structure. He also reappointed Khalid to lead the invasion, but Khalid died before the invasion commenced so command was given to Abdullah ibn Uthman.
From Nahavand, Nu’aym ibn Muqarin, the brother of Nouman ibn Muqarin, marched to Hamadan which he captured and then moved to besiege the city of Isfahan, the provincial capital. Having destroyed a Sassanid army nearby, the Arabs then besieged the city, which surrendered after several months. In 643 Nu’aym ibn Muqarin captured the city of Rey which was captured after a fierce fight and Qom which was captured with relative ease. However, both Hamadan and Rey rebelled after the Muslim army marched on forcing Nu’aym ibn Muqarin to return and subdue them.
Once both cities were firmly back under Muslim control Nu’aym ibn Muqarin turned northwards toward Tabaristan, south of the Caspian Sea. Here the local ruler Gil Gavbara had risen in revolt against the Sassanids. Since Yazgerd III was unable to put down the rebellion, he recognized Gil Gavbara as an independent ruler. When the forces of the Rashidun Caliphate arrived, Gil Gavbara signed a peace treaty with them and thus preserved his rule.
The first Muslim attack on the southern Persian province of Fars occurred in 638/639 CE without the permission of Umar. This invasion was defeated by the Sassanids and barely succeeded in withdrawing to Bahrain across the Persian Gulf. A new invasion was launched in 643 CE with the support of Umar and far better planning. After some initial success, this invasion ran into difficulty due to Persian resistance, the assassination of Umar by a Persian slave Abu Lu’lu’a Firuz, possibly with the help of Hormuzan, and the election of Uthman ibn Affan (c.576 or 579- 656CE) as the new Caliph.
The election was contested with Ali, again a prime contender being passed over, which would have greater repercussions in the future. In the aftermath of Umar’s assassination Abu Lu’lu’a Firuz, his daughter, Hormuzan, and a Christian man named Jufayna were executed, though their exact roles in the conspiracy are debated.
Following the election of Uthman, the conquest of Southern Persia resumed. Fars proved far more difficult to conquer than other parts of Persia. The local governors fought hard to maintain their independence and the cities of Bishapur and Estakhr proved quite rebellious. For a time, Yazgerd III tried to use Fars as a base from which to organize resistance. By 650/651 CE, the tied had turned decisively against the Persians and the cities of Etakhr, Gor, Kazerun, and Siraf fell in quick succession.
Fars was conquered, but Muslim rule remained shaky. Yazgerd III fled to the province of Kerman which fared little better. The southern provinces of Kerman and Makran were invaded in 651 CE and were conquered with relative ease. Muslim armies passed rabidly through Shiraz and Persepolis in 643 CE before entering Kerman where they defeated a Sassanid army in pitched battle.
The Muslim invasion of Northern Persia was timed to coincide with the southern invasion of Fars, Kerman, and Makran in 643 CE. Sassanid Armenia and Azerbaijan were the first targets of the invasion. The mountainous terrain of this region created difficulties as it favored the defenders. However, Umar directed a series of multipronged attacks, which allowed the Muslim armies to bypass and overrun the defenses of both regions. This strategy, which had already been employed several times in the past, once again proved highly effective at overcoming local resistance. By the time of Umar’s death in late 644 CE, almost all of the Southern Caucuses had been conquered so that both Sassanid Armenia and Azerbaijan were now part of the Rashidun Caliphate.
Khorasan was the second largest province of the Sassanid Empire and by 650 CE was, along with Sakastan, the only part not yet conquered by the Muslims. The invasion of Khorasan, therefore, began in 650 CE, while Sakastan was invaded in 651 CE. Yazgerd III had fled to this region after his defeats in the west but quickly alienated the local elites. His demands for tax revenue and continued resistance were not well received, Herat in Southern Khorasan was captured after a siege of several months, and the provincial capital of Merv fell without a fight.
In Sakastan the provincial capital of Zrang was surrendered after Sassanid forces suffered another crushing defeat on the battlefield. Yazgerd III gathered his final army near Balkh where he hoped to receive assistance from his Tang Chinese and Turkish allies. Neither the Tang Chinese nor the Turks were willing to commit to war with the Rashidun Caliphate by this point. So, when Yazgerd III finally faced off with Muslim forces under Ahnaf ibn Qais at the Battle of the Oxus (651 CE), the result was a crushing Sassanid defeat.
The Persian Rebellions
After the Battle of the Oxus, Yazgerd III escaped into Chinese territory. Yet he did not give up on his hope of driving out the Muslims and reviving the Sassanid Empire. As such, Yazgerd III made several cross-border forays to try and raise further Sassanid armies. By this point, he was essentially a fugitive and in late 651 CE, he was murdered by a miller near Merv. The murder is usually attributed to the miller’s desire to steal Yazgerd III’s purse or jewelry, but some claim he was an assassin sent by the governor of Merv whom Yazgerd III had deeply offended. Yazgerd III was interred by Nestorian Christian monks and was remembered in folklore as a martyred prince and despite his many personal, military, and political failures, many rulers in Islamic Iran claimed descent from him.
After the death of Yazgerd III, centralized Persian resistance to the Rashidun Caliphate and Muslim rule were effectively ended. Yazgerd III’s son, Peroz III, and grandson, Narsieh, remained active in Tang China. For many years they tried they would continue to try and raise support for a campaign to reclaim the Sassanid Empire. However, they were largely ineffective. The greatest threat to Muslim rule came from the local Persian nobility and the common people. During Uthman’s reign as Caliph, almost all of the former Sassanid territories rose in revolt. The main rebellions were in Armenia, Azerbaijan, Fars, Sakstan, Khorasan, and Makran. Several former Sassanid vassal states now under Muslim rule also rebelled. Uthman was forced to dispatch multiple expeditions to put down these revolts, which was often done with great bloodshed.
The Aftermath of the Sassanid Empire’s Fall
The Arab Conquest of the Sassanid Empire brought to an end a 400-year period of Persian rule. Culturally, politically, economically, and even religiously the territories of the former Sassanid Empire would exercise a profound influence on the new Islamic empire of the Rashidun Calpihate. The long and continued resistance of the Sassanids exacerbated many tensions between Arab and non-Arab Muslims that have persisted to this day. It brought to the surface tensions between the leadership of the early Islamic community. Uthman was assassinated in 656 CE, which led to the election of Ali, who now had to lead a polarized community. His attempts to bridge the various divides ultimately failed, resulting in the first Fitna or Islamic Civil War (656-661 CE) which ended with Ali’s assassination. This conflict would eventually split the Islamic community into the Shia and Sunni branches.
Although the Sassanid Empire was conquered, the Persians themselves became Islamized, not Arabized. Persians remained Persians and created their own distinct form of Iranian Islam, sometimes known as Islam-I Ajam. This was, however, a process that took many centuries. Yazgerd III’s son, Peroz III (c.636-679 CE), and grandson, Narsieh, spent many years trying to raise an army with which they could reconstitute the Sassanid Empire. The last futile attempt by a direct descendant of Yazgerd III to revive the Sassanid Empire ended in failure in 729 CE at the futile siege of Kamarja.
Many other Nobles and common people welcomed the Rashidun Caliphate, converted to Islam, and served as administrators, soldiers, poets, and craftsmen. They provided a technical expertise that was critical to the growth of the Caliphate and the spread of Islam. As such, the fall of the Sassanid Empire was undoubtedly one of the most important accomplishments of the Arab Conquest.