i.e., the Victorious, the second caliph of the house of Al Abbas, succeeded his brother Abul Abbas Al Saffah in the year 753, of the Hegira 136, and in the following year was inaugurated at Al Hashemiyah. Although Al Saffah had declared him presumptive heir of the crown, and he had been proclaimed caliph in the imperial city of Anbar, yet immediately upon his inauguration his uncle Abdallah ibn Ali had sufficient interest to cause himself to be proclaimed caliph at Damascus. In Arabia, Syria, and Mesopotamia, he collected a numerous army, and arrived at the banks of the Masius, near Nisibis, where he encamped, ready to dispute his royal accession by arms. Almansur collected an immense army in Persia, Khorassan, and Irak, and gave the command of it to Abu Moslem, who harassed his uncle's troops for five months, and at last totally defeated him, A.D. 754. Notwithstanding the services which Abu Moslem had rendered to the family of Al Abbas, after this victory he became an object of jealousy, and was assassinated in the presence of Almansur himself, by his express order. After the death of Abu Moslem, the standard of rebellion was raised by Simon, a Magian, who seized on the treasures of the deceased governor of Khorassan, and excited the people of that country to a general revolt; but this insurrection was suddenly quelled by the general of Almansur, Jamhur ibn Morad. The caliph avariciously seized the spoils of this victory, which so incensed Jamhur that he immediately turned his arms against his royal master; but he was soon defeated by the caliph's forces. The patriarch of Antioch was about this time detected in an illicit correspondence with the Greek emperor, and consequently was banished into an obscure part of Palestine; and in the mean time the Christians in the dominions of the caliph were prohibited from building or repairing any churches, and also were laid under several other severe restraints.

Almansur sent a large army into Cappadocia in the year 757, fortified the city of Malatia or Melitene, and deposited in it a great part of his treasures. But in this year he was attacked by a sect of believers in the metempsychosis, called the Rawanduans. This sect assembled at Al Hashemiyah, the residence of the caliph, and, by the ceremony of going in procession round his palace, intimated their purpose of invoking him as a deity, and paying him divine homage. Incensed by their impiety, the caliph ordered several of these sectaries to be imprisoned, which roused their resentment, and led them to form the design of his assassination. The generous interposition, however, of Maan ibn Zaidet, an Omynian chief, who had been under the necessity of concealing himself from the caliph's resentment, defeated their intention. This insult, received in his capital, induced him to build the city of Baghdad, and to fix his residence there, A.D. 762. In the preceding year a plan was formed to dethrone him; but it being discovered, he severely punished all who were either directly or indirectly concerned in it. He set out on a pilgrimage to Mecca in the year 774, and being seized on the road with a dangerous disease, he sent for his son and intended successor Al Mohid, and gave him some salutary advice. "I command you," said he, "to treat publicly your relations with the greatest marks of distinction, since this conduct will reflect no small degree of honour and glory upon yourself. Increase the number of your freedmen, and treat them with all kindness, as they will be of great service to you in your adversity; but neither this nor the other injunction will you fulfil. Enlarge not that part of your capital erected on the eastern bank of the Tigris, as you will never be able to finish it; but this work I know you will attempt. Never permit any of your women to intermeddle in affairs of state, or to have any influence over your councils; but this advice I know you will not take. These are my last commands; or, if you please, my dying advice; and to God I now recommend you." In parting they both gave vent to their feelings in a flood of tears. He pursued his journey to Bir-Maimun, i.e., the well of Maimun, where he died in the 63d year of his age and 20th of his reign. His remains were interred at Mecca.