This article contends that the caliph al-Mutaim came to power as the result of Abbasid opposition to his predecessor al-Mamūn whose rule is generally considered the ‘Golden Age’ of the caliphate. This opposition had already started during the Civil War between al-Amīn and al-Mamūn and it continued during the latter’s reign after winning the Civil War. In the end, al-Mamūn was able to quell the opposition by accepting al-Mutaim, at a later stage, as a member of his own inner circle. Nevertheless, ultimately al-Mamūn’s reign ended in a victory for the anti-Mamūn wing of his family. With this change, the Mamūnite vision of a strong caliphal institution passed away and henceforth the Abbasid caliphate commenced its slow decline — confirmed later by the lack of an efficient central authority and the ensuing chaos. Thus, the collapse of the Abbasid dynasty-caliphate started much earlier — after the first three decades of the ninth century CE and not during the second half of that century, contrary to what most textbooks on Islamic history state.