In the year 809
CE, the
legendary caliph Harun al-Rashid set out on a
campaign against the
Kharijis of Khurasan. The Kharijis, having played a major part in every other major
conflict in
Islamic history up to that point, were not
content to be hunted down by the caliph's forces, with whom they were
interminably at war. They fought back and killed al-Rashid, creating the need for a new caliph.
Harun al-Rashid had two sons, al-Amin and al-Mamun. Al-Amin was a man of questionable moral character, favoring drunken debauchery. It was likely that he would be easily controlled by his ministers in the event of his succession to the caliphate. He also had the backing of his mother, a noblewoman of sorts by the name of Zubaydah. Not so with al-Mamun, the a dead Persian slave girl's son who was both much more intelligent and more independent than al-Amin. In accordance with what are now known as the Meccan documents, al-Amin was the rightful ruler of the Islamic world, but the rightful place in succession of al-Mamun was debatable. Al-Mamun was satisfied to let his brother rule for the time being with the question of succession to be settled later. He went to Khurasan to establish his estate.
Then, in 810, al-Amin took the advice of his viziers and had his son Musa announced in the Friday prayers as being the next in line for the caliphate. This move was not a good one for Islamic unity, as it brought up the question of al-Mamun's place in the succession. He was summoned to Baghdad, the seat of the caliphate at the time, but he refused to go, probably out of fear of his political enemies there in the form of his brother the caliph. He was then declared a rebel by the caliph in order that he might be killed without any major backlash from the community. Al-Amin sent forces under the command of 'Ali ibn Isa to Khurasan to eliminate his brother al-Mamun, who defended with an army under the command of Tahir ibn Husayn. Al-Mamun's forces defeated al-Amin's, killing the general, 'Ali. Al-Amin refused to believe that his forces had been beaten and sent a second army against al-Mamun which met with an identical fate. Tahir's army then conquered the al-Jibal province in 811 in the name of al-Mamun while in distant Syria the people rose in revolt, briefly occupying the forces of the caliph there until 812.
In Baghdad, the son of the general 'Ali ibn Isa who had been killed by the forces of al-Mamun deposed al-Amin in 812 and declared Baghdad for al-Mamun. Unfortunately, though, this was not to be the end of the conflict, as al-Amin managed to restore himself to power. By this time, al-Mamun's troops had occupied Khuzistan and were marching on Baghdad, though. His two generals, Harthamah ibn A'yan and Tahir, laid siege to the city. In 813, Harthamah promised the caliph safe passage when the city fell, but Tahir felt differently. His men killed the caliph when they overran the city, establishing al-Mamun as caliph in his stead.
A few groups took advantage of the chaos and discontent that ran rampant in the empire at this time to raise their own revolts. A Shi'i revolt in Kufah started in 814 and managed to gain the support of the Meccans and Basrans, but Harthamah was sent to deal with it and did so without much event. Harthamah was then killed in 816 by the vizier, whose motives remain cloudy but were probably associated with Harthamah's relatively large power base and its potential threat to the caliphate. At about this same time, a man by the name of Babak raised a revolt in Azerbaijan in the name of Iranian unity and identity which was not put down for a long time to come.
Al-Mamun was Khurasani at heart and left for an extended stay there starting in 816. Baghdad society, which had never really liked al-Mamun due to his independence and unwillingness to bow before them, convinced a man by the name of Ibrahim to declare himself caliph. The city of Baghdad declared for him, putting al-Mamun in a tough spot regarding maintenance of his power. He felt that his vizier had warned him poorly and probably also had suspicions as far as the vizier's motives were concerned. In 818, the vizier met his end, although the caliph's involvement therein is inconclusive.
One way or another, though, al-Mamun moved west from Khurasan toward Baghdad to reclaim power. The rival caliph Ibrahim submitted without a fight on his arrival, allowing al-Mamun to take back power in time to face down a Khariji revolt in Khurasan as well as movements on the part of the Copts of Egypt, the Iranians of Azerbaijan, and the Arabs of the Jazirah area. Al-Mamun was able to maintain his power base and exercise effective control over areas relatively close by, but in distant places like Khurasan and the Arabian peninsula, his governors were able to practice something that was quite like autonomous leadership throughout most of his reign. In general, local loyalties began to prevail over Islamic ones as a whole. In many ways, the fourth fitnah was the war that signalled the downfall of the High Caliphate's ongoing goal of a unified Islamic world. The seeds of local loyalties were planted during this conflict and over the next forty years or so the dar al-Islam would undergo many further fragmentations that would forever shatter the possibility of its existence as an empire under one government.
Once more, the facts in this article were all taken from Marshall G. S. Hodgson's The Venture of Islam, v.1, The Golden Age of Islam.