Following the death of Harun in 809, al-Abbas was initially employed by his successor al-Amin to act as an emissary to al-Ma'mun in Khurasan, but in the midst of the growing conflict between the two brothers he was soon persuaded to switch sides and offer his allegiance to al-Ma'mun instead. In March 812 he participated in the failed coup of al-Husayn ibn Ali ibn Isa, in which he was responsible for personally apprehending both al-Amin and his mother Zubaydah bint Ja'far, and he was subsequently selected to lead the first pilgrimages undertaken in al-Ma'mun's name, in 812, 813 and 814.