Mansur, al- (äl-mänsŏrˈ) [key] [Arab., = the victorious], d. 775, 2d Abbasid caliph (754–75) and founder of the city of Baghdad. His name was in full Abu Jafar abd-Allah al-Mansur. He was brother and successor of Abu al-Abbas. A vigorous and dominating caliph, he successfully consolidated his empire even though it was threatened by internal strife and foreign wars. He could not prevent the secession of Muslim Spain, however, under the Umayyad prince Abd ar-Rahman I. Mansur lived at first, as his brother had, near Kufa, but in 762 he began to build a new city, Baghdad.