 In 816, to bolster his flagging prestige, al-Ma 'mun assumed the title "God's Caliph". Taking note of the widespread Alid support in his western provinces, al-Ma 'mun not only spared the lives of the various Alid anti-caliphs, but on 24 March 817 also named the Alid Ali ibn Musa al-Rida, a third son of Musa al-Kadhim, as his heir apparent, and even changed the official dynastic colour from Abbasid black to Alid green. Although the seriousness of al-Ma'mun's commitment to the Alid succession is uncertain — there are suggestions that Ali al-Rida was so old that he could hardly be expected to actually succeed al-Ma'mun — its impact was disastrous: not only did it fail to produce any tangible popular support, but also provoked an uproar among the members of the Abbasid dynasty in Baghdad. 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. Instead, on 17 July 817 the members of the Abbasid family in Baghdad nominated a new Caliph of their own, Harun al-Rashid's younger brother Ibrahim. Ibrahim received broad backing from the Baghdad elites, from Abbasid princes like al-Ma'mun's younger brother Abu Ishaq (the future Caliph al-Mu'tasim, r. 833 – 842), to old-established members of the bureaucracy like Fadl ibn al-Rabi (who returned to his office as hajib), and leaders of the abnaʾ. As the scholar Mohamed Rekaya commented, "in other words, it was a revival of the war between the two camps [Baghdad and Khurasan], dormant since 813".
