The Third Fitna (Arabic: الفتنة الثاﻟﺜـة, romanized: al-Fitna al-thālitha), was a series of civil wars and uprisings against the Umayyad Caliphate. It began with a revolt against al-Walid II in 744, and lasted until 747, when Marwan II emerged as the victor. The war exacerbated internal tensions, especially the Qays–Yaman rivalry, and the temporary collapse of Umayyad authority opened the way for Pro-Alid, Kharijite and other anti-Umayyad revolts. The last and most successful of these was the Abbasid Revolution, which began in Khurasan in 747, and ended with the overthrow of the Umayyad Caliphate and the establishment of the Abbasid Caliphate in 750.

The civil war began in 744 with the overthrow of al-Walid II (r. 743–744) who had succeeded his uncle Hisham ibn Abd al-Malik (r. 724–743). Hisham had been appointed by his brother, Yazid II (r. 720–724), who had specified that his son, al-Walid II, should succeed him. 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/Yamani) 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 Yamani predecessor, Khalid al-Qasri, to death. However, adherence was not clear cut, and men from both sides of the divide joined the other.

In April 744, Yazid III, a son of al-Walid I (r. 705–715), 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 (r. 717–720). 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 (shūrā), 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 Yaman, 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 (r. 744–750), the grandson of Marwan I (r. 684–685) and governor of al-Jazira (Upper Mesopotamia).

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.

Following Yazid III's death in 744, Marwan marched into Syria, claiming that he came to restore the throne to al-Walid II's two imprisoned sons. The local Qaysis of the northern districts of Qinnasrin and Hims flocked to his banner. At some point, on the road leading from Baalbek to Damascus, Sulayman, who had previously been flogged and imprisoned by al-Walid II, confronted Marwan II. With him, Sulayman had the Kalbis of southern Syria and the Dhakwāniyya, his 5,000 men strong personal army, maintained from his own funds and estates and recruited mostly from the mawālī (non-Arab Muslims). Marwan II defeated Sulayman who fled to Damascus. Marwan II forced the prisoners he had taken in the battle to pledge allegiance to al-Walid II's sons, whereupon the sons were killed by Yazid ibn Khalid al-Qasri on Sulayman's orders, along with Yusuf al-Thaqafi. Sulayman and his adherents, including the caliph-designate Ibrahim, then fled to Palmyra. Marwan II entered Damascus peacefully at the end of November or December and was declared caliph. Marwan avoided reprisals and followed a conciliatory policy, allowing the Syrian districts (junds) to choose their own governors. Soon Sulayman and Ibrahim came to Damascus and submitted to Marwan II.

Marwan II's hold on power appeared to be stabilizing, but when he moved the capital of the caliphate from Damascus to the military city of Harran it was seen as an abandonment of Syria and the move sowed resentment among the defeated Kalbis. Consequently, in summer 745, the Kalbis of Palestine rose in revolt under the local governor, Thabit ibn Nu'aym. The revolt quickly spread across Syria, even to ostensibly loyal Qaysi areas like Hims. Marwan II had to return to Syria and suppress the revolt city by city. After forcing Hims to surrender, he relieved Damascus from its siege by Yazid ibn Khalid al-Qasri, who was killed. He then rescued Tiberias, which was being besieged by Thabit. Thabit fled and his troops scattered to the winds but this three sons, Nu'aym, Bakr, and Imran were caught and executed. Thabit was later caught together with another son, Rifa'a, and executed. Following Marwan II's attack on the Kalbi's stronghold Palmyra, the Kalbi leader Abrash al-Kalbi also surrendered.

With Syria apparently back in his grip, Marwan II ordered the members of the Umayyad family to gather around him and named his two sons as his heirs. He then focused his attention on Iraq, where an army led by Yazid ibn Umar ibn Hubayra was trying to gain control of the province for him. Marwan II assembled a new army and sent it to aid Ibn Hubayra's. Meanwhile, another rebellion led by the previously defeated Sulayman had broken out in northern Syria. At Rusafa, Marwan II's army deserted to Sulayman's side. It then took Qinnasrin, and once again many Syrians dissatisfied with Marwan II joined them. Marwan II brought the bulk of his forces from Iraq and defeated Sulayman's rebels near Qinnasrin. Sulayman again escaped to Palmyra, and thence flee to Kufa. Most of his surviving troops withdrew to Hims under the command of his brother Sa'id, where they were soon besieged by Marwan II's forces. The siege lasted through the winter of 745–746, but in the end Hims surrendered. Enraged at the repeated Syrian revolts despite his earlier leniency, Marwan II now, in the summer of 746, moved to prevent any further resistance by tearing down the walls of most important Syrian towns, including Hims, Damascus and possibly also Jerusalem.