Al-Mu'tasim, Abbasid caliph (b. 796)
Abū Isḥāq Muḥammad ibn Hārūn al-Rashīd (Arabic: أبو إسحاق محمد بن هارون الرشيد; October 796 – 5 January 842), better known by his regnal name al-Muʿtaṣim biʾllāh (المعتصم بالله, lit. 'He who seeks refuge in God'), was the eighth Abbasid caliph, ruling from 833 until his death in 842. A younger son of Caliph Harun al-Rashid (r. 786–809), he rose to prominence through his formation of a private army composed predominantly of Turkic slave-soldiers (ghilmān, sing. ghulām). This proved useful to his half-brother, Caliph al-Ma'mun, who employed al-Mu'tasim and his Turkish guard to counterbalance other powerful interest groups in the state, as well as employing them in campaigns against rebels and the Byzantine Empire. When al-Ma'mun died unexpectedly on campaign in August 833, al-Mu'tasim was thus well placed to succeed him, overriding the claims of al-Ma'mun's son al-Abbas.

Al-Mu'tasim continued many of his brother's policies, such as the partnership with the Tahirids, who governed Khurasan and Baghdad on behalf of the Abbasids. With the support of the powerful chief qādī, Ahmad ibn Abi Duwad, he continued to implement the rationalist Islamic doctrine of Mu'tazilism and the persecution of its opponents through the inquisition (miḥna). Although not personally interested in literary pursuits, al-Mu'tasim also nurtured the scientific renaissance begun under al-Ma'mun. In other ways, his reign marks a departure and a watershed moment in Islamic history, with the creation of a new regime centred on the military, and particularly his Turkish guard. In 836, a new capital was established at Samarra to symbolize this new regime and remove it from the restive populace of Baghdad. The power of the caliphal government was increased by centralizing measures that reduced the power of provincial governors in favour of a small group of senior civil and military officials in Samarra, and the fiscal apparatus of the state was more and more dedicated to the maintenance of the professional army, which was dominated by Turks. The Arab and Iranian elites that had played a major role in the early period of the Abbasid state were increasingly marginalized, and an abortive conspiracy against al-Mu'tasim in favour of al-Abbas in 838 resulted in a widespread purge of their ranks. This strengthened the position of the Turks and their principal leaders, Ashinas, Wasif, Itakh, and Bugha. Another prominent member of al-Mu'tasim's inner circle, the prince of Ushrusana, al-Afshin, fell afoul of his enemies at court and was overthrown and killed in 840/1. The rise of the Turks would eventually result in the troubles of the 'Anarchy at Samarra' and lead to the collapse of Abbasid power in the mid-10th century, but the ghulām-based system inaugurated by al-Mu'tasim would be widely adopted throughout the Muslim world.

Al-Mu'tasim's reign was marked by continuous warfare. The two major internal campaigns of the reign were against the long-running Khurramite uprising of Babak Khorramdin in Adharbayjan, which was suppressed by al-Afshin in 835–837, and against Mazyar, the autonomous ruler of Tabaristan, who had clashed with the Tahirid governor of Khorasan and risen up in revolt. While his generals led the fight against internal rebellions, al-Mu'tasim himself led the sole major external campaign of the period, in 838 against the Byzantine Empire. His armies defeated Emperor Theophilos and sacked the city of Amorium. The Amorium campaign was widely celebrated, and became a cornerstone of caliphal propaganda, cementing al-Mu'tasim's reputation as a warrior-caliph.