[ثلجَة]

The Sunni branch believes that the first four caliphs--Mohammed's successors--rightfully took his place as the leaders of Muslims. They recognize the heirs of the four caliphs as legitimate religious leaders. These heirs ruled continuously in the Arab world until the break-up of the Ottoman Empire following the end of the First World War.

Shiites, in contrast, believe that only the heirs of the fourth caliph, Ali, are the legitimate successors of Mohammed. In 931 the Twelfth Imam disappeared. This was a seminal event in the history of Shiite Muslims. According to R. Scott Appleby, a professor of history at the University of Notre Dame,"Shiite Muslims, who are concentrated in Iran, Iraq, and Lebanon, [believe they] had suffered the loss of divinely guided political leadership" at the time of the Imam's disappearance. Not"until the ascendancy of Ayatollah Ruhollah Khomeini in 1978" did they believe that they had once again begun to live under the authority of a legitimate religious figure.

(Source: historynewsnetwork.org/article/934)