Abu Tahir Isma'il, better known by his regnal name al-Mansur Billah, was the third caliph of the Fatimid Caliphate in Ifriqiya, ruling from 946 until his death. He presided over a period of crisis, having to confront the large-scale Kharijite rebellion of Abu Yazid. He succeeded in suppressing the revolt and restoring the stability of the Fatimid regime.

The Fatimid Caliphate or Fatimid Empire was a caliphate extant from the tenth to the twelfth centuries CE under the rule of the Fatimids, an Isma'ili Shia dynasty. Spanning a large area of North Africa and West Asia, it ranged from the western Mediterranean in the west to the Red Sea in the east. The Fatimids trace their ancestry to the Islamic prophet Muhammad's daughter Fatima and her husband Ali, the first Shia imam. The Fatimids were acknowledged as the rightful imams by different Isma‘ili communities as well as by denominations in many other Muslim lands and adjacent regions. Originating during the Abbasid Caliphate, the Fatimids conquered Ifriqiya and established the city of al-Mahdiyya. The Fatimid dynasty ruled territories across the Mediterranean coast and ultimately made Egypt the center of the caliphate. At its height, the caliphate included—in addition to Egypt—varying areas of the Maghreb, Sicily, the Levant, and the Hejaz.