The Hashimiyya movement (a sub-sect of the Kaysanites Shia), led by the Abbasid family, overthrew the Umayyad caliphate.
The Abbasids were members of the Hashim clan, rivals of the Umayyads, but the word "Hashimiyya" seems to refer specifically to Abu Hashim, a grandson of Ali and son of Muhammad ibn al-Hanafiyya.
According to certain traditions, Abu Hashim died in 717 in Humeima in the house of Muhammad ibn Ali, the head of the Abbasid family, and before dying named Muhammad ibn Ali as his successor.
This tradition allowed the Abbasids to rally the supporters of the failed revolt of Mukhtar, who had represented themselves as the supporters of Muhammad ibn al-Hanafiyya.

Beginning around 719, Hashimiyya missions began to seek adherents in Khurasan.
Their campaign was framed as one of proselytism (dawah).
They sought support for a "member of the family" of Muhammad, without making explicit mention of the Abbasids.
These missions met with success both among Arabs and non-Arabs (mawali), although the latter may have played a particularly important role in the growth of the movement.

Around 746, Abu Muslim assumed leadership of the Hashimiyya in Khurasan.
In 747, he successfully initiated an open revolt against Umayyad rule, which was carried out under the sign of the black flag.
He soon established control of Khurasan, expelling its Umayyad governor, Nasr ibn Sayyar, and dispatched an army westwards.
Kufa fell to the Hashimiyya in 749, the last Umayyad stronghold in Iraq, Wasit, was placed under siege, and in November of the same year Abul Abbas as-Saffah was recognized as the new caliph in the mosque at Kufa.
At this point Marwan mobilized his troops from Harran and advanced toward Iraq.
In January 750 the two forces met in the Battle of the Zab, and the Umayyads were defeated.
Damascus fell to the Abbasids in April, and in August, Marwan was killed in Egypt.

The victors desecrated the tombs of the Umayyads in Syria, sparing only that of Umar II, and most of the remaining members of the Umayyad family were tracked down and killed.
When Abbasids declared amnesty for members of the Umayyad family, eighty gathered to receive pardons, and all were massacred.
One grandson of Hisham, Abd al-Rahman I, survived, escaped across North Africa, and established an emirate in Moorish Iberia (Al-Andalus).
In a claim unrecognized outside of al-Andalus, he maintained that the Umayyad Caliphate, the true, authentic caliphate, more legitimate than the Abbasids, was continued through him in Córdoba.
It was to survive for centuries.

Previté-Orton argues that the reason for the decline of the Umayyads was the rapid expansion of Islam.
During the Umayyad period, mass conversions brought Persians, Berbers, Copts, and Aramaics to Islam.
These mawalis (clients) were often better educated and more civilised than their Arab overlords.
The new converts, on the basis of equality of all Muslims, transformed the political landscape.
Previté-Orton also argues that the feud between Syria and Iraq further weakened the empire.
The Hashimiyya movement (a sub-sect of the Kaysanites Shia), led by the Abbasid family, overthrew the Umayyad caliphate. The Abbasids were members of the Hashim clan, rivals of the Umayyads, but the word "Hashimiyya" seems to refer specifically to Abu Hashim, a grandson of Ali and son of Muhammad ibn al-Hanafiyya. According to certain traditions, Abu Hashim died in 717 in Humeima in the house of Muhammad ibn Ali, the head of the Abbasid family, and before dying named Muhammad ibn Ali as his successor. This tradition allowed the Abbasids to rally the supporters of the failed revolt of Mukhtar, who had represented themselves as the supporters of Muhammad ibn al-Hanafiyya.
Beginning around 719, Hashimiyya missions began to seek adherents in Khurasan. Their campaign was framed as one of proselytism (dawah). They sought support for a "member of the family" of Muhammad, without making explicit mention of the Abbasids. These missions met with success both among Arabs and non-Arabs (mawali), although the latter may have played a particularly important role in the growth of the movement.
Around 746, Abu Muslim assumed leadership of the Hashimiyya in Khurasan. In 747, he successfully initiated an open revolt against Umayyad rule, which was carried out under the sign of the black flag. He soon established control of Khurasan, expelling its Umayyad governor, Nasr ibn Sayyar, and dispatched an army westwards. Kufa fell to the Hashimiyya in 749, the last Umayyad stronghold in Iraq, Wasit, was placed under siege, and in November of the same year Abul Abbas as-Saffah was recognized as the new caliph in the mosque at Kufa. At this point Marwan mobilized his troops from Harran and advanced toward Iraq. In January 750 the two forces met in the Battle of the Zab, and the Umayyads were defeated. Damascus fell to the Abbasids in April, and in August, Marwan was killed in Egypt.
The victors desecrated the tombs of the Umayyads in Syria, sparing only that of Umar II, and most of the remaining members of the Umayyad family were tracked down and killed. When Abbasids declared amnesty for members of the Umayyad family, eighty gathered to receive pardons, and all were massacred. One grandson of Hisham, Abd al-Rahman I, survived, escaped across North Africa, and established an emirate in Moorish Iberia (Al-Andalus). In a claim unrecognized outside of al-Andalus, he maintained that the Umayyad Caliphate, the true, authentic caliphate, more legitimate than the Abbasids, was continued through him in Córdoba. It was to survive for centuries.
Previté-Orton argues that the reason for the decline of the Umayyads was the rapid expansion of Islam. During the Umayyad period, mass conversions brought Persians, Berbers, Copts, and Aramaics to Islam. These mawalis (clients) were often better educated and more civilised than their Arab overlords. The new converts, on the basis of equality of all Muslims, transformed the political landscape. Previté-Orton also argues that the feud between Syria and Iraq further weakened the empire.
Umayyad Caliphate