So again, today, someone asks me this:
From this it is clear that the elder son of the Imam should be the next Imam, according to the Ismaili docerine. So why is Imam Hussein the Imam after Hazrat Ali, instead of Hazrat Hassan?When Hasan bin Sabbah was yet in Cairo in 471/1078, De Lacy O'Leary writes in "A Short History of the Fatimid Khalifate" (London, 1923, p. 209) that, "At the time, it appears, the court was divided into two factions over the question of the succession, the one party holding to the Khalif's elder son Nizar, the other to a younger son named Musta'li. In one place Nasir-i Khusaro says that the Khalif told him that his elder son Nizar was to be his heir, and the succession of the older son would be in accordance with the doctrines of the sect as already proved by their adherence to Ismail, the son of Jafar as-Sadiq. But Badr and the chief officials were on the side of the younger son Musta'li."
from http://www.ismaili.net/html/
Ya Ali Madad