The Hasanids, led by the brothers Muhammad (called "the Pure Soul") and Ibrahim, rejected the legitimacy of the Abbasid family's claim to power.
Reacting to mounting persecution by the Abbasid regime, in 762 they launched a rebellion, with Muhammad rising in revolt at Medina in September and Ibrahim following in Basra in November.
Ibrahim's rebellion had achieved some initial successes in southern Iraq, but his camp was torn by dissent among rival Shi'a groups as to the prosecution of the war and future political objectives.
[7][8][9] Muhammad was often called "the Pure Soul" (al-Nafs al-Zakiyya) for his noble character, but he was also "a somewhat unworldly, even romantic, individual" (Hugh N. Kennedy) and apparently less capable or learned than his younger brother Ibrahim.
Kufa, the traditional Alid base, was kept under close surveillance by the Abbasid government, and the two brothers resolved to launch a simultaneous rebellion in Basra and Medina.
Al-Mansur countered by invoking the pre-Islamic tradition of inheritance, which gave priority to a man's male relatives over his daughters—implying that the Caliphate had passed to the Abbasid line by right.
[7] Isa ibn Musa, the Caliph's nephew, was sent against Medina with 4,000 men, but Muhammad refused to abandon the holy city and insisted on meeting the Abbasid attack there.
[11][13] Ibrahim's revolt at first met with quick success, securing control over Ahwaz, Fars and Wasit, and his army register (diwan) was said to number 100,000 names.
[13] Ibrahim now was faced with a choice: a group of dedicated Alid supporters, which had managed to escape from Kufa, urged him to march on the city, while the Basrans preferred to stay in place and reach a negotiated settlement.
The Husaynids refused to take part in an uprising, while Ibrahim quarrelled with the Zaydi branch on everything from political objectives and leadership to the tactics to be followed or the provisioning of their troops.
[14][15] Al-Mansur in the meantime used his time more effectively: he mobilized troops in Syria and Iran and brought them to Iraq, and recalled Isa ibn Musa from Medina to lead them.
Although al-Ma'mun (r. 813–833) at one point nominated an Alid, Ali ibn Musa al-Rida, as his heir during the Fourth Fitna, this move was not followed up, and under Ma'mun's successors, the two families became completely estranged.