He had remained governor of the province until the Ṣaffāḥ died; when the Ṣaffāḥ passed away he laid claim to the caliphate for himself. Al-Manṣūr therefore dispatched Abū Muslim al-Khurāsānī against him, and Abū Muslim routed him. ʿAbd Allāh fled to his brother Sulaymān ibn ʿAlī, the governor of Basra, where he hid for a time; once al-Manṣūr discovered his whereabouts he had him brought to Baghdad and placed in prison.
In the present year al-Manṣūr resolved to perform the ḥajj. He summoned his nephew ʿĪsā ibn Mūsā—who, by the Ṣaffāḥ’s testament, had been the designated heir after al-Manṣūr—handed over to him his uncle ʿAbd Allāh ibn ʿAlī, and said: “This man is my enemy and yours. Kill him while I am away and do not hesitate.” Al-Manṣūr then set out for the pilgrimage, writing to ʿĪsā repeatedly from the road to spur him on: “What have you done about the matter I entrusted to you?”
When ʿĪsā ibn Mūsā took custody of his uncle he was perplexed and consulted members of his household. A number of those with sound judgement advised: “Your best course is not to kill him. Keep him with you, but announce that you have done away with him. We fear that the caliph will one day demand him openly; if you reply, ‘I killed him,’ he will order legal retaliation (qiṣāṣ). You will claim that he instructed you in secret, but you will be unable to prove it, and he will have you executed in his stead. Al-Manṣūr’s real aim is to rid himself of both of you at once.”
At this ʿĪsā changed his mind: he concealed his uncle yet declared publicly that he had put him to death. When al-Manṣūr returned from the ḥajj he directed the members of his household to enter and intercede with him on behalf of ʿAbd Allāh ibn ʿAlī. They all came in, pleaded insistently, and he consented. He then summoned ʿĪsā ibn Mūsā and said: “These people have interceded for ʿAbd Allāh ibn ʿAlī, and I have granted their request; hand him over to them.”
ʿĪsā replied, “Where is that ʿAbd Allāh? I killed him as you ordered.”
Al-Manṣūr said, “I never ordered that,” and denied having given any such instruction. ʿĪsā produced the letters al-Manṣūr had written to him time and again on the matter, but the caliph insisted that he had meant nothing of the sort, while ʿĪsā persisted in asserting that he had carried out the deed. Al-Manṣūr thereupon ordered that ʿĪsā ibn Mūsā be executed in retribution for ʿAbd Allāh. The Banū Hāshim led him out to be put to death.
When the sword was drawn ʿĪsā said, “Take me back to the caliph.” They returned him to al-Manṣūr, and he said, “Your uncle is alive; I did not kill him.”
The caliph answered, “Produce him, then; bring him here.” Al-Manṣūr found himself cornered. He ordered ʿĪsā confined in a house whose walls were built of salt; that night he had water poured over the walls, and they collapsed upon him so that he perished.
Al-Manṣūr next deposed ʿĪsā ibn Mūsā from the succession and advanced his own son Muḥammad al-Mahdī in his place. He would seat al-Mahdī above ʿĪsā on his right, never so much as turning toward ʿĪsā thereafter, and subjected him to humiliation in matters of audience, consultation, entering and leaving his presence. He went on distancing, threatening, and intimidating him until ʿĪsā voluntarily abdicated, pledging allegiance to Muḥammad ibn al-Manṣūr. In return al-Manṣūr granted him something in the order of twelve million dirhams.
From that point the standing of ʿĪsā ibn Mūsā and his sons improved with the caliph, who now showed him favour after long ignoring him. Before this there had been an abundance of correspondence and negotiation between them to pave the way for the bayʿah to al-Mahdī. The common folk would compare no one with al-Mahdī, and the commanders and notables felt the same. Al-Manṣūr pressed ʿĪsā relentlessly until he yielded, albeit under compulsion, whereupon the caliph compensated him as mentioned. The pledge of allegiance to al-Mahdī then spread throughout the lands, east and west, near and far, and al-Manṣūr rejoiced