 Tahir's unexpected victory was decisive: al-Ma'mun's position was secured, while his main opponents, the abnaʾ, lost men, prestige and their most dynamic leader. 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. Al-Amin now desperately tried to bolster his forces by alliances with Arab tribes, notably the Banu Shayban of Jazira and the Banu Qays of Syria. The veteran Abd al-Malik ibn Salih was sent to Syria to mobilize its troops along with Ali ibn Isa's son, Husayn. However, al-Amin's efforts failed due to the long-standing intertribal divisions between Qaysis and Kalbis, the Syrians' reluctance to get involved in the civil war, as well as the unwillingness of the abnaʾ to cooperate with the Arab tribes and to make political concessions to them. These failed efforts to secure Arab tribal support backfired on al-Amin, as the abnaʾ began to doubt whether their interests were best served by him. In March 812, Husayn ibn Ali led a short-lived coup against al-Amin in Baghdad, proclaiming al-Ma 'mun as the rightful Caliph, until a counter-coup, led by other factions within the abnaʾ, restored al-Amin to the throne. Fadl ibn al-Rabi, however, one of the main instigators of the war, concluded that al-Amin's case was lost and resigned from his court offices. 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.
