The Third Fitna was a series of civil wars and uprisings in the Islamic community. It followed the death of the twelfth Umayyad caliph Yazid III in 744 and lasted for about six years. The war culminated with the overthrow of the Umayyad Caliphate and the establishment of the Abbasid Caliphate in 750. Following the death of Yazid III in 744 and the abdication of his successor Ibrahim the same year, Marwan II became the sole ruler of the caliphate.
Background
After the death of the tenth Umayyad caliph Hisham ibn Abd al-Malik () in February 743, he was succeeded his nephew al-Walid II (). Al-Walid II's accession was initially well received due to Hisham's unpopularity and his decision to increase army pay, but the mood quickly changed. Al-Walid II was reported to have been more interested in earthly pleasures than in religion, a reputation that may be confirmed by the decoration of the so-called "desert castles" (including Qusayr Amra and Khirbat al-Mafjar) that have been attributed to him. His accession was resented by some members of the Umayyad family itself, and this hostility deepened when he designated his two underage sons as his heirs and flogged and imprisoned his cousin, Sulayman ibn Hisham. Further opposition arose through his persecution of the Qadariyya sect, and through his involvement in the ever-present rivalry between the northern (Qaysi/Mudari) and southern (Kalbi/Yemeni) tribes. Like his father, al-Walid II was seen as pro-Qays, especially after his appointment of Yusuf ibn Umar al-Thaqafi as governor of Iraq, who tortured his Yemeni predecessor, Khalid al-Qasri, to death. It must be noted though that adherence was not clear cut, and men from both sides of the divide joined the other.
Usurpation of Yazid III
In April 744, Yazid III, a son of al-Walid I (), entered Damascus. His supporters, bolstered by many Kalbis from the surrounding region, seized the town and proclaimed him caliph. Al-Walid II, who was at one of his desert castles, fled to al-Bakhra near Palmyra. He mustered a small force of local Kalbis and Qaysis from Hims, but when Yazid III's far larger army under Abd al-Aziz ibn al-Hajjaj ibn Abd al-Malik arrived, most of his supporters fled. Al-Walid II was killed, and his severed head was sent to Damascus. A pro-Qaysi uprising in Hims followed, under the Sufyanid Abu Muhammad al-Sufyani, but its march on Damascus was decisively defeated by Sulayman who had been released from prison. Abu Muhammad was thrown in prison in Damascus along with al-Walid II's sons.
During his brief reign, Yazid III was an exemplary ruler, modelling himself on the pious Umar II (). He was favourably disposed to the Qadariyya, and consciously tried to disassociate himself from the frequent criticism of autocratic rule leveled at his Umayyad predecessors. He promised to refrain from abuses of his power—mostly concerning widespread resentment at heavy taxation, the enrichment of the Umayyads and their adherents, the preference given to Syria over other parts of the Caliphate, and the long absence of soldiers on distant campaigns—and insisted that not only was he chosen by the community in an assembly (), rather than appointed, but that the community had the right to depose him if he failed in his duties or if they found someone more fit to lead them. At the same time, his reign saw the renewed ascendancy of the Yemenis, with Yusuf ibn Umar dismissed and imprisoned after trying, without success, to raise the Qaysis of Iraq into revolt. Yusuf's successor in Iraq and the East was the Kalbi Mansur ibn Jumhur, but he was soon replaced by the son of Umar II, Abdallah ibn Umar ibn Abd al-Aziz. During his brief tenure, Mansur tried to dismiss the governor of Khurasan, Nasr ibn Sayyar, but the latter managed to maintain his post. Yazid III died in September 744 after a reign of barely six months. Apparently due to the advice of the Qadariyya, he had appointed his brother, Ibrahim, as his successor, but he did not enjoy much support and was immediately faced with the revolt of Marwan II (), the grandson of Marwan I () and governor of al-Jazira (Upper Mesopotamia).
Rise of Marwan II
Reportedly, Marwan II, who for several years had supervised the campaigns against the Byzantines and the Khazars on the Caliphate's northwestern frontiers, had considered claiming the caliphate at the death of al-Walid II, but a Kalbi rebellion had forced him to wait. Instead, Yazid III appointed him governor to Upper Mesopotamia and he took up residence in the Qays-dominated city of Harran.