[6] The Arab invasion of Iran or the conquest of Iran by the Muslims began in 633 CE during the caliphate of Abu Bakr, reached its peak during Omar and during the Uthman period leading to the complete fall of the Sassanid government in 651 CE (30 AH) and the killing of Yazdegerd III, the last Sassanid king.
Iran conquest was the beginning of a gradual process of conversion of Iranians to Islam which lasted several centuries.
[24][25] The Arab domination over the Iranians was getting heavier and heavier, and the meaning of the Arab religion was replaced with the personal intentions and the rule of the dominant ethnic group over the defeated ethnic group and humiliation of the non-Arab people was getting acute and no progress was made for the natives.
[26] There were three groups of people at this time: Due to Al-Mutawakkil (the 10th Abbasid caliph) administrative, financial and military policies, including excessive spending, instability of administrative officials and relocation of the capital to Samarra, as well as his religious turn to the Hanbali, the caliphate was weakened and with his assassination by Turk slaves in 861 CE, the ground for the decline of Abbasid power was provided.
[citation needed] With the civil war in Baghdad and Samarra over a decade after Al-Mutawakkil's death, which led to the assassination of four caliphs, the Abbasid empire was actually fragmented and relatively independent dynasties emerged by local military powers called "Amir" in various parts of the Islamic lands.