The division between Sunni and Shiite Muslims is more political than religious but still fuels violent conflict across the Middle East.
It all started with the death of the Prophet Muhammad in 632 and a split over who should succeed him as the political and spiritual leader of the fast-growing Muslim people. One group believed that the leadership should stay in Muhammad's family, namely his cousin and son-in-law Ali. The other group backed Abu Bakr, a close friend of the prophet and the father of one of his wives [source: Harney].
Muhammad's father-in-law Abu Bakr won out, and the disgruntled supporters of Ali became known as the Shia, short for shiaat Ali or "partisans of Ali." Those who supported Abu Bakr eventually became known as Sunni or the followers of Sunnah, the prophetic tradition. Ali did get a chance to rule briefly as the fourth caliph or Muslim leader, but the feud between the sects was rekindled when one of Ali's sons, Hussein, was killed by a Sunni caliph in Iraq in 680 [source: The Economist].
The religious differences between Sunni and Shiite Muslims are less about doctrine than the source of that doctrine. All Muslims believe in the Five Pillars and the six Articles of Faith, but Sunnis view the Quran and the Sunnah (the prophetic tradition) as the definitive and final sources of enlightenment. Shiites, on the other hand, believe that Allah's revelations continued after Muhammad through a line of imams or holy leaders, and that it continues today in the form of Shiite religious leaders like the ayatollahs of Iran [source: The Economist].
Today, 80 percent of the world's Muslims are Sunnis, and in many places both sects live together in harmony. Yet much of the conflict in the Middle East is still fueled by power struggles between Sunni and Shiite-led forces. (In the Middle East and North Africa, 40 percent of Sunnis don't recognize Shias as fellow Muslims [source: Pew Research Center].)
Most observers agree that both Sunni and Shiite governments fuel sectarian resentments among the people to win support for pursuing political and military goals.