A member of the
Banu al-Abbas, Ishaq was the son of
Musa ibn Isa and grandson to
Isa ibn Musa. He was a third cousin of the caliphs
al-Amin (),
al-Ma'mun (), and
al-Mu'tasim (). In 814 Ishaq was appointed to the Yemen by al-Ma'mun, and he arrived in the province that summer. He remained in the Yemen until the following year, when he learned that nearby
Mecca had been occupied by the
Alid rebel
Hasan ibn Husayn ibn al-Aftas. Ishaq responded by organizing an expedition to recover the city for the caliph and began marching for the
Hijaz, but he suffered an attack by
Bedouin en route and was forced to retreat back to
Sana'a. A short time later, he learned that another Alid,
Ibrahim ibn Musa al-Kadhim, was advancing toward the Yemen to take control of the country, and decided not to offer any resistance to the rebels. He therefore departed, together with all of his cavalry and infantry, along the
Najd Road, effectively surrendering the province to Ibrahim. Following his departure from the Yemen, Ibrahim headed in the direction of Mecca, where
Muhammad ibn Ja'far al-Sadiq had been proclaimed as anti-caliph, and eventually reached
Mushash, where he encamped. There he was joined by his mother and several Abbasid loyalists who managed to escape from the city, and he proceeded to engage the rebels in battle for several days. After failing to defeat the Alid forces, he decided to withdraw and make his way toward
Iraq, but along the way he was met by reinforcements led by
Warqa' ibn Jamil and convinced to turn back around. The combined forces met the rebels at Bi'r Maymun near Mecca and scored a victory, putting the rebels to flight after two days of combat. Muhammad ibn Ja'far then decided to evacuate Mecca, allowing Warqa' and Ishaq to enter it in early 816 and reestablish government control. Ishaq may have subsequently led the
pilgrimage of 817. ==Notes==