The Fourth Fitna or Great Abbasid Civil War is a conflict between the brothers al-Amin and al-Ma'mun over the succession to the Abbasid Caliphate's throne.

Their father, Harun al-Rashid, had named al-Amin as the first successor, but had also named al-Ma'mun as the second with Khurasan granted to him as an appanage, while a third son, al-Qasim, had been designated as third successor.

After Harun dies in 809, al-Amin succeeds in Baghdad.

Encouraged by the Baghdad court, al-Amin begins trying to subvert the autonomous status of Khurasan; Qasim is quickly sidelined.

In response, al-Ma'mun's seeks the support of the provincial elites of Khurasan, and makes moves to assert his own autonomy.

As the rift between the two brothers and their respective camps widens, al-Amin declares his own son Musa to be his heir, and assembles a large army.

Al-Amin's troops march towards Khurasan, but al-Ma'mun's general Tahir ibn Husayn defeats them in the Battle of Rayy, and then invades Iraq and besieges Baghdad itself.

The city falls after a year, al-Amin is executed, and al-Ma'mun becomes Caliph, but he remains in Khurasan and does not come to Baghdad.This allows the power vacuum, which the civil war has created in the Caliphate's provinces, to grow, and several local rulers sprang up in Jazira, Syria and Egypt.

In addition, the pro-Khurasani policies followed by al-Ma'mun's powerful chief minister, al-Fadl ibn Sahl, and al-Ma'mun's espousal of an Alid succession, alienates the traditional Baghdad elites, who see themselves increasingly marginalized.

Consequently, al-Ma'mun's uncle Ibrahim is proclaimed rival Caliph at Baghdad in 817, forcing al-Ma'mun to intervene personally.

Fadl ibn Sahl is assassinated and al-Ma'mun leaves Khurasan for Baghdad, which he enters in 819.

The next years are taken up with consolidating al-Ma'mun's authority and reincorporating the western provinces, a process that is not completed until 827.

Some local rebellions, however, notably that of the Khurramites, drag on for far longer.