Abu Muslim was a major supporter of the Abbasid cause and was a personal friend of al-'Abbas, the future Caliph. He observed the revolt in Kufa in 736 tacitly, and worked to consolidate his position in Khurasan. With the death of the Umayyad Caliph Hisham in 743, the Islamic world was launched into civil war. Abu Muslim, who became the Abbasid governor of Khurasan after the Umayyad leadership was deposed, gained fame as a general in the late 740s in defeating the peasant rebellion of Bihafarid. Bihafarid was the leader of a syncretic Persian sect that blended Shi'ism and Mazdaism. Abu Muslim received support in suppressing the rebellion both from purist Moslems and Zoroastrians. In 750, Abu Muslim became leader of the Abbasid army and defeated the Umayyads on the Greater Zab. Abu Muslim stormed Damascus, the capital of the Umayyad caliphate, later that year.
His heroic role in the revolution and military skill, along with his conciliatory politics toward Shi'i, Sunnis, Zoroastrians, Jews, and Christians made him extremely popular among the people. Although it appears that al-'Abbas trusted him implicitly as a close confidante, al-'Abbas's brother al-Mansur (r. 754-775), apparently did not.
Under uncertain circumstances, Abu Muslim was executed on al-Mansur's orders in 755, stabbed to death in Khurasan. His murder was not well-received by either the residents of Khurasan or the people of Iran, and there was resentment among Persians over the brutal methods used by al-Mansur.
An Iranian cleric, Sunpadh, revitalized the syncretic heresy that Abu Muslim had defeated in 750. Preaching around Nishapur, Sunpadh believed that Abu Muslim was not dead, but lived on in the company of the mahdi, and would return again. The dogma received wide support among Iranian Shi'i, and outbreaks appeared in Rayy, Herat, and Sistan. A mysterious prophet named al-Muqanna, the Veiled One, appeared at about the same time, preaching that the spirit of God had appeared in Muhammad, 'Ali, and Abu Muslim, and that he would return to rule the world. The Mahdist heresies entrenched even private Shi'i beliefs in Iran that would persist until the sixteenth century when the Safavid heresy became fully pronounced.