`Abbasid caliph, reigned from 248-252/862-866, grandson of al-Mu`tasim.
Upon the death of his cousin, al-Muntasir, in 248/962, al-Musta`in was thrust into the role of caliph by the Turkish commanders in Samarra. It is said that prior to his ascension to the caliphate he was a manuscript copyist. Unrest broke out over al-Musta`in's rise in Samarra, with his opponents supporting al-Mu`tazz, which resulted in a great deal of blood shed and financial expenditure. Upon his recognition as caliph, al-Musta`in confirmed Muhammad b. `Abd Allah b. Tahir as governor of Baghdad. He then bought all the property of al-Mu`tazz and his brother al-Mu`ayyad and had both of them arrested. The Turkish guard wanted to have them killed, but they were protected by the secretary Ahmad b. al-Khasib, who was later disgraced and banished to Crete. After the caliph's army was defeated by the Byzantines in 249/863, a rebellion broke out which was put down by the vizier Utamish and the two Turkish generals Wasif and Bugha al-Saghir (Utamish was soon murdered at Bugha's instigation). al-Musta`in fled Samarra for Baghdad in 251/865, causing al-Mu'tazz's supporters to bring him out of prison in Samarra. A war broke out that only ended with the abdication of al-Musta`in in 251/866. His abdication stipulated that he move to Medina, but was detained in Wasit and later murdered in Samarra in 252/866. After his abdication, al-Musta`in had put himself under the guard of Ahmad ibn Tulun.
al-Musta`in's reign is generally seen as being characterized by a weak center. Not only is the caliph himself weak, with the Turkish vizier Utamish and his secretary Shuja` b. al-Kasim acting as true regents until Utamish's murder in 249, but there are also a number of revolts in the peripheries. Recorded revolts include anti-government outbreaks in Syria and Jordan; `Alid revolts in Kufa and at Qazvin; and the beginnings of the Zaydi movement under al-Da`i al-kabir al-Hasan b. Zayd b. Muhammad in Tabaristan and Rayy.
Zettersteen, K.V. "al-Musta`in". Trans. C.E. Bosworth. EI2.
248 - al-Musta`in becomes caliph after the death of his cousin al-Muntasir.
249 - Utamish, Wasif, and Bugha al-Saghir put down revolt. Utamish murdered by Bugha al-Saghir.
251 - al-Musta`in flees Samarra for Baghdad. al-Musta`in eventually abdicates in favor of al-Mu'tazz.
252 - al-Musta`in assassinated in Samarra.