Religion

The main divisions among Muslims are based on differing views of the political succession to Muhammad. In the Sunni view the first four caliphs (successors) were legitimately chosen by the community and were the best rulers the community has ever had. With the coming of the Umayyad dynasty, the moral level declined and rulers were sometimes corrupt, but they were usually to be accepted since the dangers of revolt were a greater evil. The Umayyad (661–750) and Abbasid (750–1258) dynasties are generally recognized by Sunnis as true caliphs. In more recent centuries the Ottoman rulers up to 1924 gained wide recognition as caliphs.

The Kharijites held that major sins disqualified a ruler, often seeking to enforce this view by violent revolts. These revolts failed, but the Kharijites contributed to Muslim theological thinking, and small groups of them continue to the present. Modern Islamic radicals have similarities to the Kharijites and are often labeled as such by their opponents.

In the Shiite view Muhammad had chosen his son-in-law and cousin, 'Ali, as his successor, but the majority passed him over three times. When he finally became caliph (fourth in the Sunni reckoning), he could not stem the moral decline and was soon murdered. After this, his rightful successors, all descendants of his, never ruled although they taught their followers. His second successor, Husayn, led a small army in revolt against the Umayyads and was killed at Karbala', in Iraq. The commemoration of his martyrdom has come to include emotional and dramatic rituals, and he has become an effective model for revolutionary action since the 1970s, especially in Iran. The largest group of Shiites, the Twelvers, recognize a line of twelve legitimate rulers, or imams, of whom the last disappeared about 874 and is expected to return in the future as the Imam Mahdi to bring just government to Earth. Sunnis also have the idea of the Mahdi as an ideal ruler, but not necessarily in the line of 'Ali, and conceptions vary considerably.

Another group of Shiites, the Seveners or Ismaili, accept a line of imams that diverges from that of the Twelvers at the seventh imam and continues with imams who are sometimes visible and sometimes hidden. Of the various subdivisions of the Ismailis, the best known in the early twenty-first century are the followers of the Aga Khan. The Ismailis constituted a significant revolutionary movement from the eleventh to the thirteenth centuries and became known to Westerners as the Assassins.

Another group of Shiites is the Zaydis, whose line of imams diverges with the fifth. Of all Shiites, the Zaydis are the closest to the Sunnis in their ideas and practices. A Zaydi imam ruled Yemen from 901 to 1961.