Muslim conquest of Persia

• 1. Introduction

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  Between 633 and 651, the Rashidun Caliphate conquered the Sasanian Empire, ending a dynasty that had ruled Persia for centuries. This was no single battle. It was eighteen years of campaigns, sieges, raids, and rebellions, fought across deserts, mountains, and river valleys from Mesopotamia to the edge of China.

  The target was not weak by accident. The Sasanian Empire still held fortified cities, a professional army, and a bureaucracy that could mobilize regional resources. Yet within four years of the execution of shah Khosrow II in 628, ten royal claimants rose and fell, and a civil war left the government decentralized by 632.

  How did a confederation of Arab tribes, only recently united, topple one of the great powers of the ancient world? Why did the Parthian families who had won victories against Byzantium refuse to fight for the Sasanian throne? And what happened to the religion, the language, and the people of Persia once the garrison towns went up? The answers run from a torn letter in a royal court to a miller who killed a king for his purse.

• 2. A Power Already Cracking

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  The Byzantine-Sasanian War of 602 to 628 had drained Persia before the first Arab raid. Khosrow II, called Parviz, first crushed Bahram Chobin's rebellion, then turned on his Byzantine enemies. From 612 to 622 he pushed Persian borders almost to their Achaemenid extent, capturing Egypt and Palestine. The conquest of Palestine was assisted by a Jewish army.

  Heraclius reversed it all. The Byzantines regrouped in 622, defeated Khosrow at the Battle of Nineveh in 627, recaptured Syria, and drove into Mesopotamia. By 629 the general Shahrbaraz agreed to peace, and the border returned to where it had stood in 602. The empire had spent two decades of blood for nothing.

  Sasanian society itself carried strain. It was divided into four classes: priests, warriors, secretaries, and commoners. The commoners formed the bulk of the population and were its sole tax base, and its poorest. As Khosrow II's conquests peaked, taxes rose sharply and most people could not pay. Ruined trade routes and a strained administration left provincial landholders, the dehqans, gaining power as central authority slipped to the generals.

  The Plague of Sheroe struck in 627 to 628, one of several epidemics carried back by armies returning from Constantinople, Syria, and Armenia. It killed many and weakened the empire further. When Khosrow II was executed in 628, ten kings and queens followed in four years. The last was Yazdegerd III, a grandson of Khosrow II, said to be a child of just eight years old.

• 3. The Confederation That Walked Away

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  Parvaneh Pourshariati's 2008 study, Decline and Fall of the Sasanian Empire, recast the traditional story. Her central thesis holds that the Sasanian Empire was not a unified state but a confederation with the Parthians, who kept a high level of independence. When that bond broke, Persia could not mount a cohesive defense.

  The powerful northern and eastern Parthian families, the kust-i khwarasan and the kust-i adurbadagan, withdrew to their strongholds and made peace with the Arabs, refusing to fight alongside the Sasanians. Despite recent victories over Byzantium, the Parthians simply pulled out of the confederation. The Sasanians were left ill-prepared and ill-equipped.

  Pourshariati also challenged the timeline. She argues the Arab conquest of Mesopotamia took place not in 632 to 634, after Yazdgerd III's accession, but in the period from 628 to 632. The consequence is striking. The conquest began precisely while Sasanians and Parthians were locked in internecine warfare over the succession.

  Earlier Western scholarship had less to work with. Investigators first relied on the accounts of the Armenian Christian bishop Sebeos and on Arabic accounts written some time after the events. The most significant earlier work was probably Arthur Christensen's L'Iran sous les Sassanides, published in Copenhagen and Paris in 1944.

• 4. Buffers And A Torn Letter

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  Since the 1st century BC, the Euphrates River had marked the border between the Roman, later Byzantine, and Parthian, later Sasanian, empires. Most battles and fortifications clustered in the hilly north. The vast desert to the south meant the only danger from that direction was the occasional raid by nomadic Arab tribesmen. Both empires hired buffer states to manage that threat: the Ghassanids served Byzantium, the Lakhmids served Persia.

  The buffers collapsed in 602. The Ghassanids had converted to the Monophysite form of Christianity, deemed heretical by the Byzantine Eastern Orthodox Church, and Byzantine attempts to suppress it sparked rebellion on the desert frontier. That same year, Khosrow II deposed and killed Nu'man III, the first Christian Lakhmid king, for trying to throw off Persian suzerainty. After Khosrow's assassination in 628 the Lakhmids became semi-independent, and they later agreed to act as spies for the Muslims after their defeat at the Battle of Hira by Khalid ibn al-Walid.

  Islamic tradition adds a confrontation of words. After the Treaty of Hudaybiyyah in 628, Muhammad is said to have sent letters urging rulers to convert, including one to Khosrow II carried by Abdullah Huzafah Sahmi Qarashi. The letter opened, "From Muhammad, the Messenger of God, to the great Kisra of Persia," and warned, "Embrace Islam so that you may remain safe."

  Khosrow reportedly tore the document apart, saying, "A pitiful slave among my subjects dares to write his name before mine." Muhammad's response, by tradition, was "May God likewise tear apart his kingdom." Some modern historians, notably Grimme and Caetani, doubt the episode, noting that the Sassanid court ceremony was notoriously intricate and unlikely to admit a letter from a minor regional power.

• 5. The First Push Into Mesopotamia

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  Muhammad died in June 632, and Abu Bakr took the title of Caliph at Medina. The Ridda Wars, the Wars of Apostasy, then occupied the Caliphate until March 633, ending with the whole Arabian Peninsula under his authority. Only then did expansion outward begin.

  Al-Muthanna ibn Haritha, a tribal chief of northeastern Arabia, raided Sasanian towns in Mesopotamia and collected considerable booty using the mobility of his light cavalry. His success prompted Abu Bakr to think about expansion, and the Caliph put his best general, Khalid ibn al-Walid, in command. Around the third week of March 633, Khalid set out from Al-Yamama with 10,000 men. Tribal chiefs joined with 2,000 warriors each, swelling his ranks to 18,000.

  Khalid won four consecutive battles: the Battle of Chains in April, the Battle of River in the third week of April, the Battle of Walaja the following month, where he used a double envelopment maneuver, and the Battle of Ullais in mid-May. Al-Hirah fell in the last week of May. Al-Anbar surrendered in July, and Ayn al-Tamr fell in the last week of July, putting most of present-day Iraq under Islamic control. In November he shattered assembled Persian and Christian Arab forces at the battles of Muzayyah, Saniyy, and Zumail, fought from three sides at night.

  The momentum did not hold. As the Rashidun army focused increasingly on the Byzantine Empire, Khalid was transferred to the Roman front in Syria while on his way to attack Qadissiyah. The newly conquered Mesopotamian territories were retaken by the Sasanian army, and the second invasion would have to win them again.

• 6. Qadisiyyah And The Capital

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  Yazdegerd III sought an alliance with Emperor Heraclius in 635, marrying his daughter, or by some traditions his granddaughter, to seal it. The plan was a two-front squeeze, Byzantines in the Levant and Persians in Mesopotamia, to push the Muslims out for good. Umar, the second caliph, moved to break the coordination before it could form.

  The Persian army the Arabs faced had real strengths and real flaws. Its heavy cavalry had proved effective against Roman forces, but it was too slow and regimented against agile, lightly armed Arab cavalry and foot archers. War elephants temporarily halted the Arab advance, until veterans returning from the Syrian fronts gave crucial instruction on how to counter them.

  Umar appointed Saad ibn Abi Waqqas, a respected senior officer suffering from sciatica, who left Medina in May 636 and reached Qadisiyyah in June. The Byzantine threat ended first, when Vahan attacked prematurely and was routed at the Battle of Yarmouk in August 636. Within three months Saad defeated a huge Persian army, drawn from every corner of the empire and led by its foremost generals, at the Battle of al-Qadisiyyah. That victory is largely regarded as a decisive turning point in Islam's growth.

  Ctesiphon, the Sassanid capital, fell in March 637 after a three-month siege. Saad and his companions took Babylon, Kutha, Sabat, and Bahurasir. To the south, Umar ordered Utbah ibn Ghazwan to capture al-Ubulla and Basra, cutting the Persian garrison there off from the capital.

• 7. Clearing The North And The Frontier Wish

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  After Ctesiphon, Persian armies regrouped at Jalawla, a hub for routes to Mesopotamia, Khurasan, and Azerbaijan. In April 637 Hashim ibn Utbah led 12,000 troops from Ctesiphon to victory at the Battle of Jalawla, then besieged the city for seven months until it fell. Abdullah ibn Muta'am captured Tikrit with the help of Christians, and Mosul surrendered on the condition of paying jizya, bringing all of Mesopotamia under Muslim control.

  Qa'qa pursued the escaping Persians under Mihran to Khaniqeen, 25 kilometres from Jalawla, defeated them, and chased them to Hulwan, which fell in January 638. When Qa'qa asked to push deeper, Umar refused, writing, "The fertile Suwad is sufficient for us; and I prefer the safety of the Muslims to the spoils of war." He wished for walls between the Suwad and the Persian hills.

  Hormuzan, one of the seven great chiefs of Persia, kept raiding and would not stay bound by treaty. After breaking peace twice and being reinforced by Yazdgerd III in late 640, he was defeated at Tuster, captured, and sent to Umar in Medina. Hormuzan apparently converted to Islam and became a useful adviser, and is also believed to be the mastermind behind Umar's assassination. Abu Musa then took Susa in January 641 and Junde Sabur soon after.

  The Zagros range still divided the two powers, and Umar repeated his wish: "I wish there was a mountain of fire between us and the Persians." But Yazdegerd III, now at Merv, recruited 100,000 veterans and volunteers under Mardan Shah and marched to Nahavand. The concentration forced Umar to act. Nouman ibn Muqarrin commanded, defeated the Persians at the Battle of Nahavand in December 642, and died in the action. The battle proved to be the key to Persia.

• 8. To The Edge Of China And After

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  Umar directed the full invasion from Medina, about 1500 kilometers from Persia, choosing Isfahan first as the heart of the empire and a conduit among Sasanian garrisons. He appointed several commanders, each with a separate mission, ordering that once a mission ended its commander would become an ordinary soldier under the next. He even meant to reinstate Khalid as field commander four years after dismissing him, but Khalid died in Emesa before the order could go out.

  The campaigns spread outward in coordinated prongs. Nu'man ibn Muqaarin took Isfahan, his brother Nu'aym took Rey and Qom and then Tabaristan south of the Caspian. In 643 Suhail ibn Adi subdued Kerman. In Sakastan, Rabi ibn Ziyad al-Harithi besieged the capital Zrang, whose governor Aparviz surrendered after seeing Rabi using the bodies of two dead soldiers as a chair, making peace for a tribute of 1 million dirhams and 1,000 slave boys or girls bearing 1,000 golden vessels.

  Fars resisted hardest. Estakhr fell only after a sacking in which the Arabs killed over 40,000 defenders, and Muslim control there remained shaky with several local rebellions. Ahnaf ibn Qais drove into Khorasan in 651, taking Nishapur, Herat, and Merv without a fight after Yazdegerd fled to Balkh. The remainder of his army was defeated at the Battle of Oxus River, and Yazdegerd narrowly escaped toward China. Umar ordered Ahnaf to stand down at the Oxus rather than cross.

  Umar himself was assassinated in November 644 by the Persian craftsman Abu Lu'lu'a Firuz. Uthman ibn Affan succeeded him, and under his reign nearly every former Sasanian province rebelled in turn. In 651 Yazdegerd III, the last Sassanid emperor, was killed near Merv by a local miller for his purse, ending both his dynasty and organized Persian resistance. It was the first time since the collapse of the Neo-Babylonian Empire in 539 BC that Mesopotamia was ruled again by Semitic-speaking people.

  What followed was slow transformation. The jizya replaced the heavier Sasanian poll taxes, mass conversions were neither desired nor allowed in the first centuries, and Middle Persian remained the official language until Al-Hajjaj ibn Yusuf switched Iraq's administration to Arabic. Most Persians did not become Muslim until the late 10th century, and most stayed Sunni until the Safavids forced conversion to Shia Islam in the 16th century. Zoroastrians fleeing persecution sailed east to India, where various kings took them in as refugees.