second Abbasid caliph, b. 709 / 714 (al-Humaymah, Syria), d. 7 October 775 (near Mecca, Arabia).
Al-Mansur was the son of the Abbasid Muhammad and a Berber slave girl. He was born in al-Humaymah, to where the Abbasids had emigrated from Arabia in 688.
The Abbasids encouraged rebel armies in a revolt against the regime of the Umayyad caliphs. When the head of the Abbasid family, al-Mansur's brother Ibrahim, was arrested al-Mansur fled with the family to Kufah in Iraq. When Ibrahim died in captivity the rebels declared al-Mansur's brother Abu al'Abbas as-Saffah the new caliph, effectively ending the rule of the Umayyad dynasty.
Al-Mansur participated in the establishment of Abbasid power under his brother's reign by leading armies into various parts of the country and submitting the governors to the new rule. The last Umayyad governor of Iran was promised safe conduct on giving up resistance, only to be executed on surrendering to al-Mansur.
Abu al'Abbas, the first Abbasid caliph, died in 754 after only five years on the throne, and al-Mansur thus became the second caliph. With the help of Abu Muslim, one of the leaders of the revolt against the Umayyads, he fought of a revolt by his uncle 'Abd Allah. When that was settled he eliminated another possible claim to power by murdering Abu Muslim. This was followed by more revolts during the next 15 years, which were put down militarily.
In 762 al-Mansur began with the building of his new capital Baghdad. The location was probably chosen to gain better control over the restless non-Muslim provinces. To stake the Abbasid claim on the region al-Mansur planned to use material from Ctesiphon, the ruined capital of the last Iranian dynasty.
Al-Mansur is known to have led a simple live without excesses of luxury. At the end of his reign the Arabian empire was well established under Abbasid rule. Al-Mansur died on his pilgrimage to Mecca.