The power that toppled the Sassanids came from an unexpected source. The Iranians knew that the Arabs, a tribally oriented people, had never been organized under the rule of a single power and were at a primitive level of military development. The Iranians also knew of the Arabs through their mutual trading activities and because, for a brief period, Yemen, in southern Arabia, was an Iranian satrapy.

Events in Arabia changed rapidly and dramatically in the sixth century A.D. when Muhammad, a member of the Hashimite clan of the powerful Quraysh tribe of Mecca, claimed prophethood and began gathering adherents for the monotheistic faith of Islam that had been revealed to him (see Religious Life , ch. 2). The conversion of Arabia proved to be the most difficult of the Islamic conquests because of entrenched tribalism. Within one year of Muhammad's death in 632, however, Arabia was secure enough for the Prophet's secular successor, Abu Bakr (632-634), the first caliph and the father-in-law of Muhammad, to begin the campaign against the Byzantine Empire and the Sassanid Empire.

Islamic forays into Iraq began during the reign of Abu Bakr. In 634 an army of 18,000 Arab tribesmen, under the leadership of the brilliant general Khalid ibn al Walid (aptly nicknamed "The Sword of Islam"), reached the perimeter of the Euphrates delta. Although the occupying Iranian force was vastly superior in techniques and numbers, its soldiers were exhausted from their unremitting campaigns against the Byzantines. The Sassanid troops fought ineffectually, lacking sufficient reinforcement to do more. The first battle of the Arab campaign became known as the Battle of the Chains because Iranian soldiers were reputedly chained together so that they could not flee. Khalid offered the inhabitants of Iraq an ultimatum: "Accept the faith and you are safe; otherwise pay tribute. If you refuse to do either, you have only yourself to blame. A people is already upon you, loving death as you love life."

Most of the Iraqi tribes were Christian at the time of the Islamic conquest. They decided to pay the jizya, the tax required of non-Muslims living in Muslim-ruled areas, and were not further disturbed. The Iranians rallied briefly under their hero, Rustam, and attacked the Arabs at Al Hirah, west of the Euphrates. There, they were soundly defeated by the invading Arabs. The next year, in 635, the Arabs defeated the Iranians at the Battle of Buwayb. Finally, in May 636 at Al Qadisiyah, a village south of Baghdad on the Euphrates, Rustam was killed. The Iranians, who outnumbered the Arabs six to one, were decisively beaten. From Al Qadisiyah the Arabs pushed on to the Sassanid capital at Ctesiphon (Madain).

The Islamic conquest was made easier because both the Byzantine Empire and the Sassanid Empire were culturally and socially bankrupt; thus, the native populations had little to lose by cooperating with the conquering power. Because the Muslim warriors were fighting a jihad (holy war), they were regulated by religious law that strictly prohibited rape and the killing of women, children, religious leaders, or anyone who had not actually engaged in warfare. Further, the Muslim warriors had come to conquer and settle a land under Islamic law. It was not in their economic interest to destroy or pillage unnecessarily and indiscriminately.

The caliph Umar (634-44) ordered the founding of two garrisoned cities to protect the newly conquered territory: Kufah, named as the capital of Iraq, and Basra, which was also to be a port. Umar also organized the administration of the conquered Iranian lands. Acting on the advice of an Iranian, Umar continued the Sassanid office of the divan (Arabic form diwan). Essentially an institution to control income and expenditure through record keeping and the centralization of administration, the divan would be used henceforth throughout the lands of the Islamic conquest. Dihqans, minor revenue collection officials under the Sassanids, retained their function of assessing and collecting taxes. Tax collectors in Iraq had never enjoyed universal popularity, but the Arabs found them particularly noxious. Arabic replaced Persian as the official language, and it slowly filtered into common usage. Iraqis intermarried with Arabs and converted to Islam.

By 650 Muslim armies had reached the Amu Darya (Oxus River) and had conquered all the Sassanid domains, although some were more strongly held than others. Shortly thereafter, Arab expansion and conquest virtually ceased. Thereafter, the groups in power directed their energies to maintaining the status quo while those outside the major power structure devoted themselves to political and religious rebellion. The ideologies of the rebellions usually were couched in religious terms. Frequently, a difference in the interpretation of a point of doctrine was sufficient to spark armed warfare. More often, however, religious disputes were the rationalization for underlying nationalistic or cultural dissatisfactions.

Data as of May 1988