The Abbasids, claiming descent from Muhammad's uncle Abbas, capitalized on various grievances, including discrimination against non-Arab Muslims (mawali), heavy taxation, and perceived impiety of Umayyad rulers.

[2] Led by Abu Muslim Khorasani, the rebellion united diverse groups, including Persians, disaffected Arabs, and Shi'a supporters, under the banner of restoring rule to the Prophet's family.

This victory led to the fall of the Umayyad dynasty and the establishment of Abbasid rule, marking a significant shift in the caliphate's power base from Syria to Iraq and ushering in a new era of Islamic governance.

Concurrent with this, a rebellion broke out in reaction to Marwan II's decision to move the capital from Damascus to Harran, resulting in the destruction of Homs – also in 746.

[4] Khorasan's expansive size and low population density meant that the Arab denizens – both military and civilian – lived largely outside of the garrisons built during the spread of Islam into Persia.

[4] While intermarriage with non-Arabs elsewhere in the Empire was discouraged or even banned,[6][7] it slowly became a habit within eastern Khorasan; and the Arabs began adopting Persian dress and as the two languages influenced one another, the ethnic barriers gradually eroded.

[15] At the same time, the capture and murder of the primary Shi'ite opposition figures rendered the Abbasids as the only realistic contenders for the void that would be left by the Umayyads.

[22] The Abbasids kept quiet about their identity, simply stating that they wanted a ruler from the descendant of Muhammad upon whose choice as caliph the Muslim community would agree.

The non-Arabs were not allowed to work for the government nor could they hold officer positions in the Umayyad military and they still had to pay the jizya tax for non-Muslims.

[12][35] Although converts to Islam made up roughly 10% of the native population – most of the people living under Umayyad rule were not Muslim – this percent was significant due to the very small number of Arabs.

If the new converts to Islam from non-Arab peoples stopped paying the jizya tax stipulated by the Qur'an for non-Muslims, the empire would go bankrupt.

[38] Even as the Arab governors adopted the more sophisticated Iranian methods of governmental administration, non-Arabs were still prevented from holding such positions.

[6] Non-Arabs were not even allowed to wear Arabian style clothing,[39] so strong were the feelings of Arab racial superiority cultivated by the Umayyads.

The controversial Umayyad governor Al-Hajjaj ibn Yusuf forced all the mawali who had left for cities, in order to avoid paying the kharaj tax, to return to their lands.

[50] Unlike the Alid revolts which were open and straightforward about their demands, the Abbasids along with the Hashimite allies slowly built up an underground resistance movement to Umayyad rule.

The rebel leader was assassinated by a son of Ibn Surayj in a revenge attack while at the same time, another Shi'ite revolt had begun in the villages.

The son of the remaining rebels signed the peace accord and Ibn Sayyar returned to his post in Merv in August of 747[53] – just after Abu Muslim initiated a revolt of his own.

On 9 June 747 (Ramadan 25, 129AH), Abu Muslim successfully initiated an open revolt against Umayyad rule,[11][54] which was carried out under the sign of the Black Standard.

In August 749, Umayyad commander Yazid ibn Umar al-Fazari attempted to meet the forces of al-Ta'i before they could reach Kufa.

[32][55] The son of Khalid al-Qasri – a disgraced Umayyad official who had been tortured to death a few years prior – began a pro-Abbasid riot starting at the city's citadel.

On Friday, 28 November 749, before the siege of Wasit had even finished, As-Saffah, the great-grandson of Muhammad's uncle, al-Abbas, was recognized as the new caliph in the mosque at Kufa.

On 16 January 750 the two forces met on the left bank of a tributary of the Tigris in the Battle of the Zab, and nine days later Marwan II was defeated and his army was completely destroyed.

When Abu Muslim recruited mixed Arab and Turks and Iranian officers along the Silk Road, he registered them based not on their tribal or ethno-national affiliations but on their current places of residence.

The Black Standard unfurled at the start of the revolution's open phase carried messianic overtones due to past failed rebellions by members of Muhammad's family, with marked eschatological and millennial slants.

[63] While the Umayyads had primarily spent their energy on wiping out the Alid line of the prophetic family, the Abbasids carefully revised Muslim chronicles to put a heavier emphasis on the relationship between Muhammad and his uncle.

While the Shi'ite and other rebellions at the time were all led by publicly known leaders making clear and well-defined demands, the Abbasids hid not only their identities but also their preparation and mere existence.

[64] Whoever he was, Abu Muslim built a secret network of pro-Abbasid sentiment based among the mixed Arab and Iranian military officers along the Silk Road garrison cities.

[21] These networks proved essential, as the officers garrisoned along the Silk Road had spent years fighting the ferocious Turkic tribes of Central Asia and were experienced and respected tacticians and warriors.

[10] Jews, Nestorian Christians, Zoroastrians and even Buddhists were re-integrated into a more cosmopolitan empire centered around the new, ethnically and religiously diverse city of Baghdad.

Though a class of Muslim clergy was absent for the first century of Islam, it was with the Abbasid revolution and after that the Ulama appeared as a force in society, positioning themselves as the arbiters of justice and orthodoxy.