After Mohammed's death the inevitable question arose: who should lead us now? Tensions arose almost immediately between those who supported Abu Bakr, Mohammed's father-in-law, and Ali, Mohammed's cousin. These tensions would only grow after Ali, and later Ali's son Husayn, were murdered. Soon the Islamic world was divided between those wo followed the Sunnah or custom of the majority and the "Partisans of Ali" (Shi'a Ali, later Shi'ites).

Shi'ite Moslems believe that authority in their religion rests with the Imam. The Imam is not only the political and religious leader of the community (as are the Sunni Caliphs); he is also infallible and sinless. Most Shi'ites believe that there were twelve Imams after Mohammed; the twelfth, known as the "Madhi" is hidden and will come again to usher in a new era of peace. In the Imam's absence, the task of spiritual guidance has been left to the Ayatollahs (literally, "Signs of God"). The Ayatollahs currently have a large say in the governance of Iran, as the 1979 Iranian Revolution created an "Islamic Republic" which was ruled according to the tenets of Shi'ite Islam.

But here there is yet another controversy: a minority "Seveners" movement traces its descent from the seventh Imam, Ismail; among the most famous of the Ismaili sects would be the Hashasheen, or "Assassins" of Alamut. Syria's ruler, Hafez Assad, is an Alawite; the Alawites consider themselves moderate "Sevener" Shi'a Moslems, although most Moslems consider them infidels. In 1974 the Lebanese leader of the Twelver Shi'is, Imam Musa al-Sadr, issues a legal decision saying that the Alawites were Shi'ite Muslims: Iran's Ayatollahs have not yet accepted this decision. As Assad continues to strive for Alawite recognition, it's likely that he will continue to support the Shi'ite-backed Hizbollah and otherwise curry favor with the Iranians.

The Shi'ite emphasis on the Imams is seen by some Sunnis as bordering on idolatry. Other cultural factors are at play as well. Shi'ite Islam has traditionally been less Arabic than Sunni Islam, with roots in Persia rather than Arabia. Still, most Sunnis and Shi'ites will recognize each other as Moslems: conflicts between Sunnis and Shi'ites are as often about ethnic or cultural differences as religious disagreements.

Today Iran remains the largest Shi'ite country, although there are sizeable Shi'ite populations throughout the Middle East. Iraq's populace is majority Shi'ite, although Hussein and his family are at least nominal Sunni Moslems; there are also many Shi'ites to be found in Lebanon and Syria, and a fair number of Shi'ite Kurds in Turkey.