The Fourth Fitna or Great Abbasid Civil War (809–827 CE) was a succession conflict between al-Amin and al-Ma'mun, sons of Caliph Harun al-Rashid, over the Abbasid caliphate. Upon Harun's death in 809, al-Amin succeeded him in Baghdad, while al-Ma'mun was appointed as ruler of Khurasan, an arrangement that soon led to tensions. Al-Amin's attempts to undermine al-Ma'mun's position and assert his own heir led to open conflict. Al-Ma'mun's forces, under general Tahir ibn Husayn, defeated al-Amin's army in 811 and captured Baghdad in 813, resulting in al-Amin's execution and al-Ma'mun's ascension as Caliph.

However, al-Ma'mun chose to stay in Khurasan, which, combined with his policies and the espousal of an Alid succession, alienated Baghdad's elites and sparked widespread unrest and local rebellions across the caliphate. This period saw the rise of local rulers and the outbreak of Alid uprisings. The conflict reflected deeper tensions within the Abbasid state, including Arab-Persian dynamics, the role of military and administrative elites, and succession practices.

The civil war concluded with al-Ma'mun's return to Baghdad in 819 and the gradual reassertion of central authority. The aftermath saw the reorganization of the Abbasid state, with a shift in the elite composition and the consolidation of regional dynasties. This period marked a significant transformation in the Abbasid caliphate, laying the groundwork for subsequent developments in Islamic governance and society.