The Fourth Fitna or Great Abbasid Civil War Military_operation resulted Causation from the conflict Hostile_encounter between the brothers al-Amin and al-Ma'mun over the succession Change_of_leadership to the throne of the Abbasid Caliphate .

Their father , Caliph Harun al-Rashid , had named Name_conferral al-Amin as the first successor , but had also named Name_conferral al-Ma'mun as the second , with Khurasan granted Giving to him as an appanage .

After Harun died Death in 809 , al-Amin succeeded Change_of_leadership him in Baghdad .

Encouraged Convincing by the Baghdad court , al-Amin began Process_start trying to subvert Change_of_leadership the autonomous status of Khurasan , and al-Qasim was quickly sidelined Process_end .

As the rift Quarreling between the two brothers and their respective camps widened Expansion , al-Amin declared Expressing_publicly his own son Musa as his heir and assembled Cause_to_amalgamate a large army .

In 811 , al-Amin 's troops marched Self_motion against Khurasan , but al-Ma'mun 's general Tahir ibn Husayn defeated Conquering them in the Battle Hostile_encounter of Rayy , and then invaded Attack Iraq and besieged Besieging Baghdad itself .

The city fell after a year , al-Amin was executed Killing , and al-Ma'mun became Becoming Caliph .

Historians have interpreted Protest the conflict Violence variously ; in the words of the Iranologist Elton L. Daniel , it has been regard as `` a conflict Violence over the succession between a rather incompetent , besotted Change_sentiment al-Amin and his shrewdly competent brother al-Ma'mun ; as the product Manufacturing of harem intrigues ; as an extension Expansion of the personal rivalry Competition between the ministers al-Fadl b. Rabi ' and al-Fadl b. Sahl ; or as a struggle Violence between Arabs and Persians for the control Control of the government '' .