d
put at their head Mansur, a son of Mahdi, who refused to assume the
title of caliph, but consented to be Mamun's vicegerent instead of Hasan
b. Sahl.

Meanwhile, at Merv, Mamun was adopting a decision which fell like a
thunderbolt on the Abbasids. In A.H. 201 (A.D. 817), under pretence of
putting an end to the continual revolts of the partisans of Ali, and
acting on the advice of his prime minister Fadl, he publicly designated
as his successor in the Caliphate Ali ar-Rida, a son of that Musa
al-Kazim who perished in the prison of Mahdi, a direct descendant of
Hosain, the son of Ali, and proscribed black, the colour of the
Abbasids, in favour of that of the house of Ali, green. This step was
well calculated to delight the followers of Ali, but it could not fail
to exasperate the Abbasids and their partisans. The people of Bagdad
refused to take the oath to Ali b. Musa, declared Mamun deposed, and
elected his uncle, Ibrahim, son of Mahdi, to the Caliphate.[33] It was
only indirectly that the news reached the caliph, who then saw that Fadl
had been treating him as a puppet. His anger was great, but he kept it
carefully to himself. Fadl was one day found murdered, and Ali b. Musa
died suddenly. The historians bring no open accusation against Mamun,
but it seems clear that the opportune removal of these men was not due
to chance. Mamun affected the profoundest grief, and, in order to disarm
suspicion, appointed as his prime minister the brother of Fadl, Hasan b.
Sahl, whose daughter Buran he afterwards married. Soon after the news
came to him that Hasan b. Sahl had become insane. Mamun appointed an
officer to act as his lieutenant, and wrote that he was coming to Bagdad
in a short time. From that moment the pseudo-caliph Ibrahim found
himself deserted, and was obliged to seek safety in concealment. His
precarious reign had, however, lasted nearly two years. Mamun had found
out also that the general uneasiness was largely due to his treatment of
Harthama and Tahir, the latter having been put in a rebellious country
without the men and the money to maintain his authority. The caliph
therefore wrote to Tahir to meet him at Nahrawan, where he was received
with the greatest honour. Having taken all precautions, Mamun now made
his solemn entry into Bagdad, but, to show that he came as a master, he
still displayed for several days the green colours, though at last, at
the request of Tahir, he consented to resume the black. From t