From “Amir al-Mu‘minin: History of the Caliphs” by Suleiman al-Dimashqi
The reasons for the Abbasid Revolution are many, but predominant among them was the Umayyads’ oppression of non-Arabs. The Persians, who by this point had been mostly converted to Islam, were still oppressed by their Arab Umayyad rulers. Beginning in 129 AH[747 AD], the Abbasid Revolution was centered in Persia, Mesopotamia, and Greater Khorasan[1]. The Abbasids, who descended from Al-Abbas, one of the companions of the prophet, had numerous supporters against the Umayyads. The Abbasids were supported by both Shi’ites and Sunnis in the region. They were supported by non-Arab Muslims who wished to see the end of Umayyad discrimination, and non-Muslim dhimmi[2] who wanted greater freedom to practice their religions.
Under the Caliphs As-Saffah and Al-Mansur, as well as the Persian general Abu Muslim, the Abbasid Caliphate came to rule an area stretching from Egypt in the west to Transoxiana in the east. The Abbasids established their capital in the city of Baghdad, where the old Sassanid capital of Ctesiphon had once been. Abd al-Rahman I, a member of the Umayyad dynasty in Damascus, established his own Caliphate based out of Constantinople[3]. The Caliphate of Constantinople would establish its’ rule over Antaloia, the Balkans, and the Italian peninsula. Yusuf ibn Abd al-Rahman al-Fihri, the governor of Al-Andalus, declared the new Fihrid Emirate[4].
In North Africa, between the Fihrid and Abbasid domains, a charismatic Berber leader who went by the name of Ali ibn Muhammad al-Bijāyi, rallied the Kabyle Berbers under his leadership, founding the Kharijite Caliphate of Guenzet. The Caliphs of Guenzet were elected by the Caliphate’s religious leaders based on how devout they were, in accordance with Kharijite beliefs. Since choosing one person out of the entire population to be the new Caliph would be inefficient, it became custom that only the most learned of Islamic law would be considered eligible candidates. It was also custom that a candidate would never vote for themselves, as a sign of humility. Despite its pseudo-Democratic government, the Caliphate of Gueznet was highly oppressive to both non-Muslims and non-Kharijite Muslims.
The Umayyad Caliphs of Constantinople would convert to the Mu’tazilite sect of Islam, who had a rationalistic interpretation of the Qur’an. The Fihrids were originally Sunni, but trade with the Caliphate of Constantinople would introduce Mu’tazila Islam, which would in time grow more dominant. The Abbasids were Sunni, and used their Sunnism to contrast them with the Mu’tazilite Caliphs of Constantinople and the Kharijite Caliphs of Guenzet. The divide between the Sunnis, Mu’tazilites, Kharijites, and later the Shi’ites who would rebel against the Sunni Abbasids would be a permanent split.
[1]A historical region on the Iranian Plateau between Western and Central Asia
[2]Non-Muslims within historical Islamic states who were considered “people of the book”(originally meaning Christians, Jews, and Mandaeans, but also variously applied to Zoroastrians, Samaritans, and even Hindus, Buddhists, and Jains) . Dhimmi were considered protected and allowed to practice their religion in exchange for paying a special tax called “jizya.”
[3]IOTL, Abd al-Rahman established the Emirate(later Caliphate) of Córdoba. ITTL, Constantinople proved more appealing than Al-Andalus when both were available.
[4]The Fihrids were an influential dynasty in Iberia and North Africa with a penchant for acting independently of the Caliph in Damascus. IOTL, they were deposed by the aforementioned Abd al-Rahman. ITTL, with Abd al-Rahman going for Constantinople instead of Spain, they stay in power.