In 811, al-Amin's troops marched against Khurasan, but al-Ma'mun's general Tahir ibn Husayn defeated them in the Battle of Ray, and then invaded Iraq and besieged Baghdad itself.

This allowed the power vacuum which the civil war had fostered in the Caliphate's provinces to grow, and several local rulers sprang up in Jazira, Syria and Egypt.

[3][4][5] While al-Ma'mun's origin was less prestigious than the purely Arab al-Amin, his ties to Khurasan and the Iranian-dominated eastern provinces were an important factor in his choice as heir.

[4][13][14] Indeed, the years after the fall of the Barmakids saw an increasing centralization of the administration and the concomitant rise of the influence of the abnaʾ, many of whom were now dispatched to take up positions as provincial governors and bring these provinces under closer control from Baghdad.

Although the latter now resided chiefly in what is now Iraq, they insisted on retaining control of Khurasani affairs and demanded that the province's revenues be sent west to supply their salaries, something strongly resisted by the local Arab and Iranian elites.

[16][17] His harsh taxation measures provoked increasing unrest, which expressed itself in Kharijite uprisings and, finally, a rebellion by the governor of Samarkand, Rafi ibn al-Layth.

This uprising forced Harun himself, accompanied by al-Ma'mun and the powerful chamberlain (hajib) and chief minister al-Fadl ibn al-Rabi, to travel to the province in 808.

It was in this time that al-Ma'mun came to rely upon his wazir, the former Barmakid protégé al-Fadl ibn Sahl, who began to implement a policy of conciliation and cooperation with the local elites, whose autonomy and privileges were guaranteed.

[1][4] The covenant of 802 however soon began to fall apart over Baghdad's centralizing ambitions and the dispute over the status of Khurasan: the abnaʾ, led by Ali ibn Isa, whom Harun had imprisoned but who was now set free and appointed head of the Caliph's bodyguard, were joined by other influential officials, chief amongst them al-Fadl ibn al-Rabi, in demanding that Khurasan and its revenue return to the direct control of the central government, even if that meant breaking the stipulations of the Mecca agreement.

[1][24][27] Al-Ma'mun, who could not rely on large military forces and whose position was consequently weak, was at first inclined to accede to his brother's demands, but al-Fadl ibn Sahl dissuaded him from this course and encouraged him to seek support among the native population of Khurasan, who also opposed control by the caliphal court.

[24][28] Al-Ma'mun, who was already favourably regarded after the excesses of Ali ibn Isa, consciously set about to cultivate the support of the local population, reducing taxes, dispensing justice in person, conceding privileges to the native princes, and demonstratively evoking episodes from the beginnings of the Abbasid movement in the province.

Al-Ma'mun replied by declaring himself imam, a religious title which shied of directly challenging the Caliph but nevertheless implied independent authority, as well as hearkening back to the early days of the Hashimiyya movement which had carried the Abbasids to power.

[34] Tahir now advanced westwards, defeated another abnaʾ army of 20,000 under Abd al-Rahman ibn Jabala after a series of hard-fought engagements near Hamadan, and reached Hulwan by winter.

However, al-Amin's efforts failed due to the long-standing intertribal divisions between the Qays and the Kalb, the Syrians' reluctance to get involved in the civil war,[a] as well as the unwillingness of the abnaʾ to cooperate with the Arab tribes and to make political concessions to them.

[25] At about the same time, al-Ma'mun was officially proclaimed caliph, while Fadl ibn Sahl acquired the unique title of Dhu 'l-Ri'asatayn ("he of the two headships"), signifying his control over both civil and military administration.

[34][36] As Tahir's army closed on Baghdad, the rift between al-Amin and the abnaʾ was solidified when the desperate Caliph turned to the common people of the city for help and gave them arms.

The revolt was nominally led by the Alid Ibn Tabataba, and after his death by Zayd, a son of the imam Musa al-Kadhim who had been executed in 799 on Harun al-Rashid's orders.

[31][36][47] Secondary pro-Alid movements also seized control of Yemen (under Ibrahim al-Jazzar, another son of Musa al-Kadhim) and the Tihamah, including Mecca, where Muhammad al-Dibaj, a grandson of the Alid imam Ja'far al-Sadiq, was proclaimed anti-caliph in November 815.

Hasan ibn Sahl had already been forced to abandon the city, where various factional leaders now shared power, and the news of the Alid succession ruined his attempts at conciliation.

[50] Ibrahim moved to secure control of Iraq, but although he captured Kufa, Hasan ibn Sahl, who had made Wasit his base of operations, managed to get to Basra first.

Finally, in December 817 Ali al-Ridha succeeded in revealing to al-Ma'mun the real situation in Iraq, and convinced him that the turmoil in the Caliphate was far greater than the Sahlids presented it to be, and that a reconciliation with Baghdad was necessary.

[43][50][51] Al-Ma'mun now set about to reconcile himself with the opposition: he rescinded the Alid succession, restored black as the dynastic colour, sent Hasan ibn Sahl into retirement, and recalled Tahir from his exile in Raqqa.

[54][55] At the time al-Ma'mun entered Baghdad, the western provinces of the Caliphate had slipped away from effective Abbasid control, with local rulers claiming various degrees of autonomy from the central government.

Nasr was willing to acknowledge al-Ma'mun's authority, but demanded concessions for his followers and remained hostile to the Abbasids' Persian officials, so that he had to be browbeaten into submission by a show of force before his capital, Kaysum, in 824–825.

It was not until the accession of al-Mu'tasim, who employed his new military corps composed of Turkish slave-soldiers (mawali or ghilman) against the Khurramites, that their rebellion was suppressed in 837, after years of hard campaigning.

The most tangible change was in the elites who supported the new regime: the abnaʾ, the old Arab families and the members of the Abbasid dynasty itself lost their positions in the administrative and military machinery, and with them their influence and power.

[64][65] The provinces of the Caliphate were now grouped into larger units, often controlled by a hereditary dynasty, like the Tahirids in Khurasan or the Samanids in Transoxiana, usually of Iranian descent.

[60][66] This system was further elaborated and acquired its definite characteristics in the reign of Abu Ishaq (al-Mu'tasim), who created a tightly controlled, centralized state, and expanded his Turkish corps into an effective military force with which he waged campaigns against the Byzantines and internal rebellions alike.

[68] At the same time, the willingness of al-Ma'mun and his successors to embrace the non-Arab populations of the Caliphate, especially in the Iranian East, as well as to entrust the governance of these provinces to local dynasties with considerable autonomy, helped to end a long series of religiously motivated rebellions and reconciled these populations to Islam: the rate of conversion during al-Ma'mun's reign increased markedly, and that was the time when most of the local princely families of the Iranian lands finally became Muslims.

As El-Hibri comments, "in time this development represented a prelude to the emergence of autonomous provincial dynasties in the east, which would relate to the caliphal centre in nominal terms of loyalty only".