Muslim conquest of Persia

The Arab conquest of Persia, led to the end of the Sassanid Empire in 644, the fall of the Sassanid dynasty in 651 and the eventual decline of the Zoroastrian religion in Iran. Arabs first attacked the Sassanid territory in 633, when general Khalid ibn Walid invaded Mesopotamia (what is now Iraq), which was the political and economic center of the Sassanid state. Following the transfer of Khalid to the Roman front in the Levant, the Muslims eventually lost their holdings to Iranian counterattacks. The second invasion began in 636 under Saad ibn Abi Waqqas, when a key victory at the Battle of Qadisiyyah led to the permanent end of Sassanid control west of Iran. The Zagros mountains then became a natural barrier and border between the Rashidun Caliphate and the Sassanid Empire. Owing to continuous raids by Persians into the area, Caliph Umar ordered a full invasion of the Sassanid Iranian empire in 642, which was completed with the complete conquest of the Sassanids by mid 644. The quick conquest of Iran in a series of well coordinated multi-pronged attacks, directed by Caliph Umar from Medina several thousand miles from the battlefields in Iran, became his greatest triumph, contributing to his reputation as a great military and political strategist.
Iranian historians have sought to defend 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." By 651, most of the urban centers in Iranian lands, with the notable exception of the Caspian provinces and Transoxiana, had come under the domination of the Arab armies. Many localities in Iran staged a defense against the invaders, but in the end none was able to repulse the invasion. Even after the Arabs had subdued the country, many cities rose in rebellion, killing the Arab governor or attacking their garrisons, but reinforcements from the caliphs succeeded in putting down all these rebellions and imposing the rule of Islam. The violent subjugation of Bukhara (q.v.) after many uprisings is a case in point. Conversion to Islam (q.v.) was, however, only gradual. In the process, many acts of violence took place, Zoroastrian scriptures were burnt and many mobads executed (for examples, see Balāḏori, Fotuḥ, p. 421; Biruni, Āṯār, p. 35).Once conquered politically, the Persians began to reassert themselves by maintaining Persian language and culture. Regardless, Islam was adopted by many, for political, socio-cultural or spiritual reasons, or simply by persuasion, and became the dominant religion

Historiography and recent scholarship

When Western academics first investigated the Muslim conquest of Persia, they only had to rely on the accounts of the Armenian Christian bishop Sebeos, and accounts in Arabic that were written some time after the events they describe. The most significant work was probably that of Arthur Christensen, and his L’Iran sous les Sassanides, published in Copenhagen and Paris in 1944.
However recent scholarship, both Iranian and Western,[citation needed] has begun to question the traditional narrative. Parvaneh Pourshariati, in her Decline and Fall of the Sasanian Empire: The Sasanian-Parthian Confederacy and the Arab Conquest of Iran, published in 2008, provides both a detailed overview of the problematic nature of trying to establish exactly what happened, and a great deal of original research that questions fundamental facts of the traditional narrative, including the timeline and specific dates.

Pourshariati's central thesis is that contrary to what was commonly assumed, the Sassanian Empire was highly decentralized, and was in fact a "confederation" with the Parthians, who themselves retained a high level of independence. Despite their recent victories over the Byzantine Empire, making the Byzantines a client-state of the Sassanians, the Parthians unexpectedly withdrew from the confederation, and the Sassanians were thus ill-prepared and ill-equipped to mount an effective and cohesive defense against the Muslim armies. Moreover, the powerful northern and eastern Parthian families, the kust-i khwarasan and kust-i adurbadagan, withdrew to their respective strongholds and made peace with the Arabs, refusing to fight alongside the Sassanians.

Another important theme of Pourshariati's study is a re-evaluation of the traditional timeline. Pourshariati argues that the Arab conquest of Mesopotamia "took place, not, as has been conventionally believed, in the years 632–634, after the accession of the last Sasanian king Yazdgird III (632–651) to power, but in the period from 628 to 632." An important consequence of this change in timeline means that the Arab conquest started precisely when the Sassanians and Parthians were engaged in internecine warfare over who was to succeed the Sassanian throne.

Sassanid Empire Before the Conquest

Since the 1st century BC, the border between the Roman (later Byzantine) and Parthian (later Sassanid) empires had been the Euphrates river. The border was constantly contested. Most battles, and thus most fortifications, were concentrated in the hilly regions of the north, as the vast Arabian or Syrian Desert (Roman Arabia) separated the rival empires in the south. The only dangers expected from the south were occasional raids by nomadic Arab tribesmen. Both empires therefore allied themselves with small, semi-independent Arab principalities, which served as buffer states and protected Byzantium and Persia from Bedouin attacks. The Byzantine clients were the Ghassanids; the Persian clients were the Lakhmids. The Ghassanids and Lakhmids feuded constantly—which kept them occupied, but that did not greatly affect the Byzantines or the Persians. In the 6th and 7th centuries, various factors destroyed the balance of power that had held for so many centuries.