The other major branch of Islam, which claims about 10% of the Muslim population and exists primarily in Iraq and Iran, is the more militant Shi'a. The Shi'ites, as those comprising the Shi'a sect are called, splintered from the Sunnis primarily over the question of the Caliphate. Regarding this matter, there are specifically two points of disagreement between Shi'ites and Sunnis. First, the Shi'ites place more rigid genealogical restrictions on the Caliph than do the Sunnis. On the one hand, Sunnis believe that the Caliph should be a descendent of Muhammad's tribe. On the other hand, Shi'ites argue that the Caliph should descend specifically from `Ali, Muhammad's son-in-law. In fact, the word Shi'ite means ''partisan'' and indicates that Shi'ites are ''partisans of `Ali'' (Rood, 1994). Second, the Shi'ites differ with the Sunnis regarding the authority of the Caliph. Unlike the Sunnis, Shi'ite Muslims believe that the Islamic leader, whom they call the imam, is more than merely a guardian of Muhammad's prophetic legacy. Rather, Muhammad bequeathed `Ali with his wilaya (i.e., his ''spiritual abilities''), enabling him to interpret the Qur'an and to lead the Islamic community infallibly. Though there are various interpretations, Shi'ites generally believe that the wilaya has been passed down through the subsequent generations of `Ali's descendants. They further believe that this ''cycle of the wilaya'' will continue until the last day when humankind will be resurrected and judged (see Kerr, 1982, p. 331).

The majority faction within the Shi'a branch, known as the Imamis (most of whom live in Iran), believes that the completion of the wilaya cycle will end with the messianic return of the twelfth imam. According to this sect, the twelfth imam has been in ''occultation'' (the state of hiding) since the third century of Islam. They believe, however, that the ayatollahs (senior experts in Islamic law) have access to the hidden imam, and thus, have the right to interpret Islamic law and make religious rulings (Kerr, 1982, p. 331). The late Ayatollah Khomeini, perhaps the most widely-remembered Shi'ite leader among contemporary Westerners, was considered to be the spokesman for the hidden imam.