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The Abbasid civil war of 865โ866, sometimes known as the Fifth Fitna, was an armed conflict during the "Anarchy at Samarra" between the rival caliphs al-Musta'in and al-Mu'tazz, fought to determine who would gain control over the Abbasid Caliphate. The war, which lasted for about a year, largely revolved around a prolonged siege of Baghdad and ended with al-Mu'tazz as sole caliph. Al-Musta'in was abandoned by his supporters and forced to abdicate; in spite of a guarantee that his life would be spared, he was executed shortly afterward.
The outcome of the war was a major victory for the Turkic military establishment, which had been responsible for al-Mu'tazz's rise to power, and allowed the Turks to maintain their effective power over the government and military of the caliphate. The partisans of al-Musta'in, namely the Tahirid family, the Arab military factions, and the citizens of Baghdad, continued to be excluded from the politics of the central government after their surrender, although they were allowed to keep the positions they had before the war. Central Iraq, where most of the fighting took place, was devastated by the activities of both sides.