The Third
fitnah began in 744
CE with the
death of al-Walid II at the hands of Kalb
tribesmen in
Syria. He had gravely offended them by favoring their
rival faction, the Qays, to an
inordinate degree and had also
offended the more
religiously
inclined members of the populace by
neglecting almost totally the role of
spiritual leader that was expected of him. The Kalb brought another caliph to power with the additional backing of the
piety-minded
faction that had opposed the former caliph. This caliph, Yazid III, was quite reform-minded and had the backing of the
majority of the
people. Unfortunately, though, he suffered an untimely death later in the year, bringing a much
weaker caliph to power in the form of his
brother Ibrahim.
With all the regicide, it was only a matter of time before powerful relatives of the dead caliph came seeking vengeance. Marwan II, a Qays leader and general in the ongoing fight against the Byzantine Romans, rose in rebellion in the northern areas. He managed to wipe out the caliph Ibrahim relatively quickly and had himself declared caliph after he had conquered Damascus.
Another rebellion began in Kufah under the Shi'i leader 'Abd-Allah Ibn-Mu'awiyah after Marwan had established himself reasonably well as caliph. After holding out against Marwan's troops for some time, the rebellion was forced to retreat to the west Iranian highlands, where it was eventually destroyed in 747 following an extended conflict, thanks mostly to the superior organizational skills of the Marwani troops.
Then, with the onset of the year 745, the Syrians revolted against Marwan. This rebellion was not put down until a year later, sapping many of Marwan's resources in dealing with it. The Kharijis, those perennial malcontents, decided to take advantage of the generally chaotic atmosphere and once again raised two separate revolts: one in Iraq in 745 and one in 746 in the Arabian peninsula, centered in Oman on the southeast coast. Both of these rebellions had a great deal of success, managing to once again seize large portions of the Islamic world. The Iraq-based rebellion captured a hefty portion of the Jazirah area, in which Marwan's power was based, before the caliph's forces crushed the rebellion both in the Jazirah and in Iraq in 746-747. The Kharijis of the Arabian peninsula captured Mecca itself during their final thrust westward before Marwan ejected them from the city and began to reduce their forces.
While Marwan was occupied with all of these sundry revolts, a revolt that had been going on since 734 in distant Khurasan was given direction in the form of a freed slave by the name of Abu-Muslim. He came in order to plead for the Shi'i cause in the form of vengeance on the Umayyad caliphs for their repeated killings of members of the house of Muhammad. His banners were black, symbolizing mourning for these deaths. He was secretly associated with the 'Abbasid leader Ibrahim (not to be confused with the caliph by the same name) and when in 747, one year after the defeat of the standing revolt in the area at the hands of Marwan, Abu-Muslim moved from secret agitation to open revolt, Marwan had Ibrahim killed, thinking that the death of the would-be ruler would remove the motivation for revolution. Ibrahim's name was instead added to the list of martyrs to be avenged, thereby adding momentum to the revolt. With the destruction of the rest of the opposition movements by the year 748, malcontents of many stripes were left with no recourse other than backing the 'Abbasid revolt. Since Marwan had been occupied with the other revolts of the time, he was unable to deal with this one until it had become unstoppable. Abu-Muslim defeated a major Marwani army at the Euphrates in Mesopotamia after conquering Iran almost without opposition and proclaimed Ibrahim's brother Abu-l-'Abbas caliph in Kufah in 749. At a battle by the Tigris river in 750, Marwan's entire force was decimated, leaving him with no more military force on which to draw because of his destruction of Syrian military power following their revolt. He was able to do nothing more than run in front of the conquering 'Abbasids until he was cornered in Egypt and killed. The 'Abbasids, under Abu-Muslim's leadership, destroyed his power completely and ruled for quite a long time afterwards, although Abu-Muslim himself was executed relatively quickly after the revolution at the hands of the 'Abbasids, who feared future opposition from his powerful persona.
That's a complicated fitnah.
As always, all facts and most interpretations in this article are taken from Marshall G. S. Hodgson's The Venture of Islam, v.1, The Golden Age of Islam.