[2] His great-grandfather Ruzaiq was a mawla of Talha ibn Abd Allah al-Khuza'i, an Arab nobleman from the Khuza'a tribe, who served as the governor of Sistan.

In 810, the caliph al-Amin, and his brother, Al-Ma'mun, came in conflict which each others, which later led to a civil war; in January 811, al-Amin formally began the Great Abbasid Civil War when he appointed Ali ibn Isa as governor of Khurasan, placed him at the head of an unusually large army of 40,000, drawn from an elite group known as abna′, and sent him to depose al-Ma'mun.

[4][6][7] Tahir's unexpected victory was decisive: al-Ma'mun's position was secured, while his main opponents, the abna′, lost men, prestige and their most dynamic leader.

[8] Tahir now advanced westwards, defeated another abna′ army of 20,000 under Abd al-Rahman ibn Jabala after a series of hard-fought engagements near Hamadan, and reached Hulwan by winter.

However, al-Amin's efforts failed due to the long-standing intertribal divisions between Qaysis and Kalbis, the Syrians' reluctance to get involved in the civil war, as well as the unwillingness of the abna′ to cooperate with the Arab tribes and to make political concessions to them.

[7][8] These failed efforts to secure Arab support backfired on al-Amin, as the abna′ began to doubt whether their interests were best served by him.

Fadl ibn al-Rabi, however, one of the main instigators of the war, concluded that al-Amin's case was lost and resigned from his court offices.

[7] At about the same time, al-Ma'mun was officially proclaimed caliph, while his vizier Fadl ibn Sahl acquired the unique title of Dhu 'l-Ri'asatayn ("he of the two headships"), signifying his control over both civil and military administration.

[8][10] As Tahir's army closed on Baghdad, the rift between al-Amin and the abna′ was solidified when the desperate Caliph turned to the common people of the city for help and gave them arms.