Muslim conquest of Persia

While Arabia was experiencing the rise of Islam in the 7th century, Persia was struggling with unprecedented levels of political, social, economic, and military weakness; the Sasanian army had greatly exhausted itself in the Byzantine–Sasanian War of 602–628. Following the execution of Sasanian shah Khosrow II in 628, Persia's internal political stability began deteriorating at a rapid pace. Subsequently, ten new royal claimants were enthroned within the next four years.^[4] Shortly afterwards, Persia was further devastated by the Sasanian Interregnum, a large-scale civil war that began in 628 and resulted in the government's decentralization by 632.


Amidst Persia's turmoil, the first Rashidun invasion of Sasanian territory took place in 633, when the Rashidun army conquered parts of Asoristan, which was the Sasanians' political and economic centre in Mesopotamia.^[5] Later, the regional Rashidun army commander Khalid ibn al-Walid was transferred to oversee the Muslim conquest of the Levant, and as the Rashidun army became increasingly focused on the Byzantine Empire, the newly conquered Mesopotamian territories were retaken by the Sasanian army. The second Rashidun invasion began in 636, under Sa'd ibn Abi Waqqas, when a key victory at the Battle of al-Qadisiyyah permanently ended all Sasanian control to the west of modern-day Iran. For the next six years, the Zagros Mountains, a natural barrier, marked the political boundary between the Rashidun Caliphate and the Sasanian Empire. In 642, Umar ibn al-Khattab, eight years into his reign as Islam's second caliph, ordered a full-scale invasion of the rest of the Sasanian Empire. Directing the war from the city of Medina in Arabia, Umar's quick conquest of Persia in a series of coordinated and multi-pronged attacks became his greatest triumph, contributing to his reputation as a great military and political strategist.^[4] In 644, however, he was assassinated by the Persian craftsman Abu Lu'lu'a Firuz, who had been captured by Rashidun troops and brought to Arabia as a slave.


Some Iranian historians have defended their forebears by using Arab sources to illustrate that "contrary to the claims of some historians, Iranians, in fact, fought long and hard against the invading Arabs."^[6] By 651, most of the urban centres in Iranian lands, with the notable exception of the provinces along the Caspian Sea (i.e., in Tabaristan and Transoxiana), had come under Muslim domination. Many localities fought against the invaders; although the Rashidun army had established hegemony over most of the country, many cities rose in rebellion by killing their Arab governors or attacking their garrisons. Eventually, military reinforcements quashed the Iranian insurgencies and imposed complete control. The Islamization of Iran was gradual and incentivized in various ways over a period of centuries, though some Iranians never converted and there were widespread cases of Zoroastrian scriptures being burnt and Zoroastrian priests being executed, particularly in areas that experienced violent resistance.^[7] Islam had become Iran's predominant religion by the Late Middle Ages;^[8][9] the majority of Iranians were Sunni Muslims until the Safavids forcefully converted Iran to Shia Islam in the 18th century.

Persian raids in Mesopotamia (638–641)[edit]

By February 638 there was a lull in the fighting on the Persian front. The Suwad, the Tigris valley, and the Euphrates valley were now under complete Muslim control. The Persians had withdrawn to Persia proper, east of the Zagros mountains. The Persians continued raiding Mesopotamia, which remained politically unstable. It seemed the Zagros range was the dividing line between the Rashidun Caliphate and the Sassanids. In the latter part of 638, Hormuzan, who was one of the seven great chiefs of Persia, and had commanded a corps at the Battle of Qadisiyyah, intensified his raids in Mesopotamia. Saad, on Umar's instructions, attacked Hormuzan, while Utbah ibn Ghazwan, aided by Nouman ibn Muqarin, attacked Ahvaz and forced Hormuzan into a peace treaty, under which Ahvaz would remain in Hormuzan's possession as a Muslim vassal state and would pay tribute. Hormuzan broke the treaty, leading Umar to send Abu Musa Ashaari, governor of Busra, to deal with him. After another defeat, Hormuzan signed another treaty on similar terms to the last. This peace also proved short-lived once Hormuzan was reinforced by fresh Persian troops sent by Emperor Yazdgerd III in late 640. The troops concentrated at Tuster, north of Ahvaz. Umar sent the Governor of Kufa, Ammar ibn Yasir, the governor of Busra, Abu Musa, and Nouman ibn Muqarin there, where Hormuzan was defeated, captured and sent to Umar in Medina. Hormuzan apparently converted to Islam and remained a useful adviser to Umar throughout the remainder of the Persian campaign. He is also believed to be the mastermind behind Umar's assassination in 644.


After the victory at Tustar, Abu Musa marched against the strategically important Susa in January 641, capturing it after a siege of a couple of months. Next, Abu Musa marched against Junde Sabur, the only place left of military importance in the Persian province of Khuzistan, which surrendered to the Muslims after a siege of a few weeks.^[35]