Emerging from a period of long seclusion, the leader of the burgeoning community of Ismaili Shi'i Muslims was declared the first Fatimid Imam-caliph in the year 909. Abd Allah al-Mahdi founded the only sustained Shi'i dynasty (909-1171) to rule over substantial parts of the medieval Muslim world, rivalling both the Umayyads of Spain and the Abbasids. At its peak, the Fatimid Empire extended from the Atlantic shores of North Africa, across the southern Mediterranean and down both sides of the Red Sea, covering also Mecca and Medina.

This accessible history, the first of two volumes,...