Abu Tahir Isma'il (Arabic: أبو طاهر إسماعيل, romanized: Abū Ṭāhir ʾIsmāʿīl; January 914 – 18 March 953), better known by his regnal name al-Mansur Billah (Arabic: المنصور بالله, romanized: al-Manṣūr biʾllāh, lit. 'The Victor through God'), was the thirteenth Isma'ili imam and third caliph of the Fatimid Caliphate in Ifriqiya, ruling from 946 until his death. He succeeded his father, al-Qa'im, after the latter's death, in what was likely a bloodless palace coup. At the time of al-Mansur's accession, most of the Fatimid mainland realm in Ifriqiya had been lost to a large-scale anti-Fatimid revolt led by the Kharijite preacher Abu Yazid, who was laying siege to al-Qa'im's fortified coastal palace city of al-Mahdiya. Unlike his father he was an active and publicly visible monarch, but plagued by illness, which led to his early death.
Al-Mansur immediately took up the fight against the revolt with considerable energy, but kept his father's death secret until after the final suppression of the rebellion, governing instead as the ostensible designated successor and "Sword of the Imam". Leaving the trusted eunuch chamberlain Jawdhar to run the government in his stead, al-Mansur took to the field in person, leading the Fatimid army to victory over the rebel army outside Kairouan, and pursuing its remnants into the Hodna Mountains. After a long pursuit, Abu Yazid was finally cornered and captured, before dying of his injuries on 19 August 947. After Abu Yazid's death, al-Mansur publicly proclaimed his caliphate. His victory over Abu Yazid was celebrated as a triumph over a 'False Messiah', and as heralding a new beginning for the dynasty and its divine mission. Al-Mansur spent the remainder of his reign in al-Mansuriya, a new capital city he founded in 948. During his reign, he also secured the allegiance of the Sanhaja Berbers under Ziri ibn Manad, recaptured Tahert from the rebellious Miknasa Berbers, suppressed a revolt in the overseas Fatimid province of Sicily, launched naval raids against Byzantine holdings in southern Italy, and engaged in a mounting antagonism with the rival Umayyad state of Córdoba.