His victorious general Tahir (d. 822) had stormed Baghdad and taken the head of al-Amin. al-Amin had defied Haroun al-Rashid's will of succession by saying that his own son would succeed him, whereas Haroun's will specified that al-Ma'mun would succeed al-Amin.
al-Ma'mun's record as an administrator is marked by his efforts as a humanist and toward the centralization of power and the certainty of succession. The Bayt al-Hikma, or House of Wisdom, was established during his reign. The ulama emerged as a real force in Islamic politics during al-Ma'mun's reign for opposing the mihna, which was instituted in 827.
The mihna, or 'ordeal,' was comparable to Spain's inquisition. In the effort to centralize power and test the loyalty of his subjects, al-Ma'mun required elites, scholars, judges and other government officials to undergo the test, which was a series of questions relating to theology and faith. The penalty for failing the mihna could include death.
The controversy over the mihna was exacerbated by al-Ma'mun's adoption of Mu'tazili theology. Mu'tazili theology was deeply influenced by Aristotelian thought and Greek rationalism, and stated that the understanding of the Koran was to be found on logical bases from reality and the world. This defied the mystical element of Islam that stated that understanding of God was only to be found in the Koran itself. Moreover, the Mu'tazilis stated that the Koran was created rather than eternal, in opposition to general Moslem opinion that the Koran and the Divine were coeternal. The fact that the Mu'tazili school had its foundations in the paganism of Greece further disenchanted conservative Islamic clerics.
Although al-Mahdi had proclaimed that the caliph was the protector of Islam against heresy, and had claimed too the ability to declare orthodoxy, religious scholars in the Islamic world believed that al-Ma'mun was overstepping his bounds in the mihna. The penalties of the mihna became increasingly difficult to enforce as the ulama became firmer and more united in their opposition. Although the mihna persisted through the reigns of two more caliphs, al-Mutawakkil abandoned it in 848. The failure of the mihna seriously damaged Caliphal authority and ruined the reputation of the office for succeeding caliphs. The caliph would lose much of his religious authority to the opinion of the ulama as a result of the mihna.
The ulama and the major Islamic law schools became truly defined in the period of al-Ma'mun and Sunnism, as a religion of legalism, became defined in parallel. Ibn Hanbal, the founder of the Hanbali legal school, became famous for his opposition to the mihna. al-Ma'mun's simultaneous opposition and patronage of intellectuals led to the emergence of important dialogues on both secular and religious affairs, and the Bayt al-Hikma became an important center of translation for Greek and other ancient texts into Arabic. This Islamic renaissance spurred the rediscovery of Hellenism and ensured the survival of these texts into the European renaissance.
al-Ma'mun had been named governor of Khurasan by Haroun, and after his ascension to power, the caliph named Tahir as governor for his military services in order to assure his loyalty. It was a move that al-Ma'mun soon regretted, as Tahir and his family became entrenched in Iranian politics and became increasingly powerful in the state, contrary to al-Ma'muns desire to centralize and strengthen Caliphal power. The rising power of the Tahirids became a threat as al-Ma'muns own policies alienated them and his other opponents.
The shakiriya, which were to trigger the movement of the capital from Baghdad to Samarra during al-Mu'tasim's reign, were raised in al-Ma'mun's time. The shakiriya were military units from Central Asia and North Africa hired, complete with their commanders, to serve under the Caliph.
al-Ma'mun, in an attempt to win over the Shi'a Muslems to his camp, named the eighth Imam, Ali ar-Rida, his successor, if he should outlive al-Ma'mun. Most Shi'ites realized, however, that ar-Rida was too old to survive him and saw al-Ma'mun's gesture as empty; indeed, ar-Rida died in 818. The incident served to further alienate the Shi'ites from the Abbasids, who had already been promised and denied the Caliphate by al-'Abbas.
al-Ma'mun also attempted to divorce his wife during his reign, who had not borne him any children. His wife hired a Syrian judge of her own before al-Ma'mun was able to select one himself; the judge, who sympathized with the caliph's wife, refused the divorce. Following al-Ma'mun's experience, no further Abbasid caliphs were to marry, preferring to find their heirs in the harem.
al-Ma'mun died in 833, and was succeeded by his brother, al-Mu'tasim.