Al-Mansur or Abu Ja'far Abdallah ibn Muhammad al-Mansur (95 AH – 158 AH (714 AD – 775 AD);Arabic: أبو جعفر عبدالله بن محمد المنصور‎‎) was the second Abbasid Caliph reigning from 136 AH to 158 AH (754 AD – 775 AD) and succeeding his brother Abu al-'Abbas al-Saffah. Al-Mansur is generally regarded as the real founder of the Abbasid Caliphate, one of the largest polities in world history, for his role in stabilizing and institutionalizing the dynasty. He is also known for founding the 'round city' of Madinat al-Salam which was to become the core of imperial Baghdad.

Al-Mansur was born at the home of the 'Abbasid family after their emigration from the Hejaz in 95 AH (714 CE). "His father, Muhammad, was reputedly a great-grandson of Abbas ibn Abd al-Muttalib, the youngest uncle of Mohammad; his mother, as described in the 14th-century Moroccan historical work Rawd al-Qirtas was one Sallama, "a Berber woman given to his father." He reigned from Dhu al-Hijjah 136 AH until Dhu al-Hijjah 158 AH (754 CE – 775 CE). In 762 he founded as new imperial residence and palace city Madinat as-Salam (the city of peace), which became the core of the Imperial capital Baghdad. Al-Mansur was concerned with the solidity of his regime after the death of his brother Abu'l `Abbas (later known as as-Saffah). In 754 he defeated Abdallah ibn Ali's bid for the Caliphate, and in 755 he arranged the assassination of Abu Muslim. Abu Muslim was a loyal freed man from the eastern Iranian province of Khorasan who had led the Abbasid forces to victory over the Umayyads during the Third Fitna in 749–750; he was subordinate to al-Mansur but also the undisputed ruler of Iran and Transoxiana. The assassination seems to have been made to preclude a power struggle in the empire; some findings suggest that Abu Muslim became incredulous and paranoid and that this 'necessitated' the assassination.

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