The Caliphate of Al-Ma'mun He was pledged allegiance definitively on the day his brother Al-Amin was killed. He summoned Ubayd Allah ibn Al-Mahdi, whom he had appointed as his deputy in Egypt, and entrusted the governorship to Al-Muttalib ibn Abdullah Al-Khuzai.

A few months later, he replaced him with Al-Abbas ibn Musa ibn Isa, who had ruled Egypt three times during the days of Harun Al-Rashid, taking charge of its prayers and its land tax. In the year 199 AH, Al-Abbas ibn Musa relinquished the governorship of Egypt. Al-Ma'mun sent Al-Muttalib ibn Abdullah, his predecessor, as a replacement. Shortly thereafter, he was replaced by Al-Sirri ibn Al-Hajj.

News of this spread throughout the Islamic kingdom, but the Alids, descendants of Imam Ali ibn Abi Talib, did not cease demanding their rights to the caliphate. They called upon the people to pledge allegiance to Ali ibn Musa. Al-Ma'mun accepted this, and he was in Khurasan.

He consulted his minister, Al-Fadl ibn Sahl, on the matter, and he advised him to bequeath the caliphate after his death to Ali ibn Musa, because Al-Fadl was a Shiite.

However, this policy only led to increased discord and widening rifts. The parties multiplied, and this was particularly distressing to the Abbasids, as they saw the caliphate slipping from their hands to the Alids. They revolted in Baghdad in the year 201 AH, a revolt that resulted in the deposition of Al-Ma'mun and the pledging of allegiance to Ibrahim ibn Al-Mahdi.

His authority did not extend beyond Baghdad. He was not fit to rule, his strength having waned. He fled to Samarra, where he remained for about two months, then abdicated the caliphate in the year 203 AH. Al-Ma'mun returned to Baghdad in the year 204 AH.

He entered it in a green Alid robe. A week later, the caliphate returned to the black Abbasid robes. In this year, Imam Muhammad ibn Idris, known as Al-Shafi'i, the author of Al-Shafi'i, passed away. He was fifty-four years old. Al-Sirri ibn Al-Hajj, the governor of Egypt, also passed away. His son, Muhammad ibn Al-Sirri, was pledged allegiance to him, severing ties with the Abbasids.

In the year 205 AH, Tahir ibn Al-Husayn, the chief commander of Al-Ma'mun, passed away in Merv, the capital of Khurasan. His son, Abdullah ibn Tahir, came to Egypt and resided there. In the year 206 AH, Al-Ma'mun ordered Abdullah ibn Tahir to remain in his post and withdraw from Egypt. He appointed his brother, Al-Mu'tasim, over Egypt and Syria, and gave him one hundred thousand dinars. He also gave this sum as a gift to Abdullah ibn Tahir. It is said that he also gave a similar sum to his son Al-Abbas.