The campaign was accompanied by a contingent of Byzantine captives under the renegade general Manuel the Armenian, who, given Abbas's relative inexperience, may have been the actual commander of the army.

After the Arab army crossed the Cilician Gates and took Heraclea Cybistra in early July, it divided in three corps, headed by the Caliph, al-Mu'tasim and Abbas, and proceeded to raid across Cappadocia.

[6][7] On receiving news of the Caliph's illness, Abbas abandoned his army and sped to his father's encampment, where he joined his uncle al-Mu'tasim at al-Ma'mun's deathbed.

[1] As al-Ma'mun's son, and given his prominent role in his father's last campaigns, Abbas was considered a contender for the succession, although it is unclear if he was ever formally appointed heir apparent, as al-Dinawari claims.

[2][9] Despite his acceptance of his uncle's succession, Abbas was removed from his governorship of the frontier lands, but retained the support of several leading figures in the Abbasid court.

This discontent resulted in an alleged conspiracy led by the general Ujayf ibn Anbasa which aimed to kill al-Mu'tasim and place Abbas on the throne.

He was imprisoned at Manbij by another of al-Mu'tasim's lieutenants, al-Afshin, who executed him by a slow and torturous way: Abbas was fed very salty food, denied water, and exposed to the sun wrapped in a felt blanket, until he died.