sion were forfeited, and
caused the army to swear allegiance to his own son Musa, a child of
five, on whom he bestowed the title of _an-Natiq bil-Haqq_ ("he who
speaks according to truth"), A.H. 194 (A.D. 809-810). On hearing the
news, Mamun, strong in the rightfulness of his claim, retaliated by
suppressing the caliph's name in all public acts. Amin immediately
despatched to Khorasan an army of 40,000 under the command of Ali b.
'Isa, who had regained his former influence, and told the caliph that,
at his coming to Khorasan, all the leading men would come over to his
side. Zobaida, the mother of the caliph, entreated Ali to treat Mamun
kindly when he should have made him captive. It is said that Fadl b.
Sahl had, through a secret agent, induced Fadl b. Rabi' to select Ali,
knowing that the dislike felt towards him by the Khorasanians would
double their strength in fighting against him. Mamun, on his side, sent
in all haste an army of less than 4000 men of his faithful Khorasanians,
and entrusted their command to Tahir b. Hosain, who displayed
remarkable abilities in the war that ensued. The two armies met under
the walls of Rai (Shaaban 195, May 811). By a bold attack, in the manner
of the Kharijites of yore, Tahir penetrated into the centre of the
hostile army and killed Ali. The frightened army fled, leaving the camp
with all its treasures to Tahir, who from that day was named "the man
with the two right hands." A courier was despatched immediately to Merv,
who performed the journey, a distance of about 750 miles, in three days.
On the very day of his arrival, Harthama b. A'yan had left Merv with
reinforcements. Mamun now no longer hesitated to take the title of
caliph.

When the news of Ali's defeat came to Bagdad, Amin sent Abdarrahman b.
Jabala to Hamadan with 20,000 men. Tahir defeated him, forced Hamadan to
surrender, and occupied all the strong places in Jabal (Media). The year
after, Amin placed in the field two new armies commanded respectively by
Ahmad b. Mazyad and Abdallah b. Homaid b. Qahtaba. The skilful Tahir
succeeded in creating divisions among the troops of his adversaries, and
obtained possession, without striking a blow, of the city of Holwan, an
advantage which opened the way to the very gates of Bagdad. He was here
reinforced by troops sent from Khorasan under the command of Harthama b.
A'yan, who was appointed leader of the war against Amin, with orders to
send Tahir to Ahwaz. Tahir continued hi