A branch of Islam with about 12 to 15 million followers worldwide, many in India. Their spiritual leader is the Aga Khan. The present one, Prince Karim Aga Khan IV (acceded 1957), is the forty-ninth Ismaili imam.

The Shiites trace the succession of their imams from Fatima, daughter of Muhammad, and are a minority among Muslims. The Ismailis are a minority among Shiites, tracing their imams from Ismail, eldest son of Jafar al-Sadiq, the sixth Shi'i Imam. Ismail died before his father, who died in 765, the majority of Shiites accepting the younger son Musa. Because they were such a minority, the Ismailis authorized taqiyya or tactical denial of faith in order to survive among the larger sects.

In 910 an Ismaili named Ubaydallah al-Mahdi founded a dynasty, known to history as the Fatimid Caliphate of Egypt. This survived in opposition to the older Sunni caliphate in Baghdad until Egypt was conquered by Saladin in 1171, founding the Ayyubid Sultanate.

Hassan i Sabbah, the founder of the sect of the Assassins, converted to Ismailism in 1072 and entered the impregnable fortress of Alamut in northern Persia in 1090. A disputed succession in the Fatimid caliphate in 1094 led to the Assassins taking on rival Fatimid leaders as their victims. The Assassins of Alamut were finally subjugated by the Mongol ilkhan (deputy khan) Hulagu Khan in about 1256, and the survivors were scattered.

They did not figure much until 1843 when the newly-created Aga Khan, the 46th Ismaili imam, rebelled against the Shah and was obliged to flee Persia for India. Arriving in Bombay in 1848 he accepted the allegiance of an Ismaili branch called the Khojas. To quell the disputes and dissensions this caused, the British authorities investigated the Aga Khan's genealogy and validated his claim. Since then the Aga Khans and their Ismailis have been Western-oriented. The Aga Khan III was an international diplomat, head of the League of Nations, and both he and the Aga Khan IV have been noted philanthropists. In the twentieth century Ismailis formed much of the Muslim immigrant community in East Africa.

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