In 802 Harun al-Rashid, father of al-Ma'mun and al-Amin, ordered that al-Amin succeed him, and al-Ma'mun serve as governor of Khurasan and as caliph after the death of al-Amin. Al-Ma'mun was reportedly the older of the two brothers, but his mother was a Persian woman while al-Amin's mother was a member of the reigning Abbasid family. After al-Rashid's death in 809, the relationship between the two brothers deteriorated. In response to al-Ma'mun's moves toward independence, al-Amin declared his own son Musa to be his heir. This violation of al-Rashid's testament led to a civil war in which al-Ma'mun's newly recruited Khurasani troops, led by Tahir bin Husain (d. 822), defeated al-Amin's armies and laid siege to Baghdad. In 813, al-Amin was beheaded, and al-Ma'mun recognized as caliph throughout the empire.
There were disturbances in Iraq during the first several years of al-Ma'mun's reign, while the caliph was in Merv. On November 13, 815, Muhammad ibn Ja'far al-Sadiq (Al-Dibaj) claimed the Caliphate for himself in Mecca. He was defeated and he quickly abdicated asserting that he had only become caliph on news that al-Ma'mun had died. Lawlessness in Baghdad led to the formation of neighborhood watches. When in A.H. 201 (817 AD) al-Ma'mun named Imam Reza the Eighth descendant of Muhammad his heir. This was not accepted by people in Baghdad. This was a political move by al-Ma'mun since most of Persia was sympathetic to the Hashemites. Al-Ma'mun's opponents in Baghdad gave allegiance to Ibrahim ibn al-Mahdi. His forces fought Kharijites, al-Ma'mun's [noun omitted?], and arrested the neighbourhood watch commander Sahl ibn Salamah.
Imam Reza informed al-Ma'mun of happenings in Baghdad and al-Ma'mun set out for the City of Peace on the day the fast ended, April 12, 818. At Tus, he stopped to visit his father's grave. On the last day of Safar in 203 AH, Imam Reza died. Imam Reza was buried beside the caliph's grave. Following the death of Imam Reza a great revolt took place in Khurasan, Persia. Al-Ma'mun wept and mourn for Imam Reza and tried to show himself innocent of the crime. But for all he did, he could not get himself acquitted and prove his innocence. Al-Ma'mun wrote to Hasan ibn Sahl, his governor in Iraq, the Hijaz, etc. informing him of his grief on the Imam's death. The governor fell ill and al-Ma'mun appointed Dinar ibn Abdallah to replace him. Some of Ibn al-Mahdi's commanders deserted him, and he died.
Muhammad ibn Jarir al-Tabari states that al-Ma'mun entered Baghdad on August 11, 819 (v. 32, p. 95). He wore green and had others do so. Informed that compliance with this command was despite popular opposition to the colour, on August 18 he reverted to traditional Abbasid black. While Baghdad became peaceful, there were disturbances elsewhere. In A.H. 210 (825-826) Abdullah ibn Tahir al-Khurasani secured Egypt for al-Ma'mun freeing Alexandria from Andalusians and quelling unrest. The Andalusians moved to Crete where al-Tabari records their descendants were still living in his day (see Abo Hafs Omer Al-Baloty). Abdallah returned to Baghdad in 211 Hijri or (826-827 C.E.) bringing defeated rebels with him.
Also, in 210 Hijri or the year 825-826 C.E. there was an uprising in Qum sparked by complaints about taxes. After it was quashed, the tax assessment was set significantly higher. In 212 Hijri or 827-828 C.E., there was an uprising in Yemen. In 214 (829-830) Abu al-Razi who had captured one Yemeni rebel was killed by another. Egypt continued to be unquiet. Sindh was rebellious. In 216 (831-832) Ghassan ibn Abbad subdued it. An ongoing problem for al-Ma'mun was the uprising headed by Babak Khorramdin. In 214 Babak routed a Caliphate army killing its commander Muhammad ibn Humayd.
By the time Al-Ma'mun became Caliph, the Arabs and the Byzantines were settled down into border skirmishing, with Arab raids into Anatolia replied in kind by Byzantine raids that "stole" Christian subjects of the Abbasid Caliphate and forcibly settled them into the Anatolian farmlands to increase the population and hence provide more farmers and more soldiers. The situation changed however with the rise to power of Michael II in 820 AD. Forced to deal with the rebel Thomas the Slav, Michael had few troops to spare against a small Andalusian invasion of 40 ships and 10,000 men against Crete, which fell in 824 AD. A Byzantine counter offensive in 826 AD failed miserably. Worse still was the invasion of Sicily in 827 by Berbers of Tunis. Even so, Byzantine resistance in Sicily was fierce and not without success whilst the Arabs became quickly plagued by internal squabbles. That year, the Arabs were expelled from Sicily but they were to return.
In 829, Michael II died and was succeeded by his son Theophilos. Theophilos experienced mixed success against his Arab opponents. In 830 AD the Arabs returned to Sicily and after a year-long siege took Palermo from their Christian opponents and for the next 200 years they were to remain there to complete their conquest, which was never short of Christian counters.The Caliph Al-Ma'mun meanwhile launched an invasion of Anatolia in 830 AD. Al-Ma'mun triumphed and a number of Byzantine forts were taken; he spared the surrendering Byzantines. Theophilos did not relent and in 831 captured Tarsus from the Muslims.The next year, learning Byzantines had killed some sixteen hundred people, Al-Ma'mun returned. This time some thirty forts fell to the Caliphate forces, with two Byzantine defeats in Cappadocia.
Al-Ma'mun's relations with the Byzantine Greeks is marked by his efforts in the translation of Greek philosophy and science. Al-Ma'mun gathered scholars of many religions at Baghdad, whom he treated magnificently and with tolerance. He sent an emissary to the Byzantine Empire to collect the most famous manuscripts there, and had them translated into Arabic. It is said that, had he been victorious over the Byzantine Emperor, Al-Ma'mun would have made a condition of peace be that the emperor hand over of a copy of the "Almagest".
The 'mihna', is comparable to Medieval European inquisitions only in the sense that it involved imprisonment, a religious test, and a loyalty oath. The casualties of 'Abbasid inquisition would not approach a fraction of those executed in Europe under similar circumstances. This is because the people who were subject to the mihna were traditionalist scholars whose social influence and intellectual quality was uncommonly high. Al-Ma'mun introduced the mihna with the intention to centralize religious power in the caliphal institution and test the loyalty of his subjects. The mihna had to be undergone by elites, scholars, judges and other government officials, and in consisted of a series of questions relating to theology and faith. The central question was about the createdness of the Qur'an, if the interrogatee stated he believed the Qur'an to be created, he was free to leave and continue his profession.
During his reign, alchemy greatly developed and the pioneers of the science were Jabir Ibn Hayyan and his student Yusuf Lukwa was patronized by Al-Ma'mun, although he was unsuccessful in his attempts regarding the transmutation gold, his methods greatly led to the patronization of Pharmaceuticals compounds.
Al-Ma'mun, unable to have a child due to his marital circumstances, and in an attempt to win over the Shi'a Muslims 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-Tabari (v.32, pp. 224–231) recounts how Al-Ma'mun was sitting on the river bank telling those with him how splendid the water was. He asked what would go best with this water and was told a specific kind of fresh dates. Noticing supplies arriving, he asked someone to check whether such dates were included. As they were, he invited those with him to enjoy the water with these dates. All who did this fell ill. Others recovered. But Al-Ma'mun died. As he was dying he spoke, expressing his belief in the unity of God and his reliance on God's mercy. He encouraged his successor to continue his policies and not burden the people with more than they could bear. This was on August 9, 833.
^ Lee S. Tesdell, "Greek Rhetoric and Philosophy in Medieval Arabic Culture: The State of the Research," in: Discourses of Power: Grammar and Rhetoric in the Middle Ages, ed. Carol Poster and Richard Utz (Evanston, IL: Northwestern University Press, 1999), pp. 51-58.
^ Goldschmidt, Arthur (2002). A concise history of the Middle East. Boulder, Colorado: Westview Press. pp. 78. ISBN 0813338859.