A look at the history of Shiite Islam

A look at the past and current influence of Shiite Muslims:

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HISTORY _ A dispute over succession after the Prophet Muhammad's death in 632 split the Muslim world into Sunni and Shiite branches. Shiites believe Ali, the prophet's son-in-law, was Muhammad's rightful heir to lead the new faith. Sunnis believe the succession went to Abu Bakr, the prophet's close associate. Both branches follow the same basic tenets, including acceptance of the Quran as sacred text and sharing nearly all traditions based on the life of Muhammad and companions. But important differences include commemorations of rival historical figures, and the Shiite practice of giving greater authority to top clerics to interpret the Quran and traditions.

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POPULATION _ Most conservative estimates place the Shiite population at no more than 15 percent of the world's 1.2 billion Muslims, but some areas where Shiites live, including Afghanistan and Pakistan, have been accused of providing unreliable census data. Shiites are the overwhelming majority in Iran and the dominant branch of Islam in Iraq and Bahrain. Shiite communities are also mainly found in Lebanon, Azerbaijan, the Gulf States and India.

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TENSIONS _ In Iraq, Sunni Arabs oppose proposals for a federal system, which they claim would give too much self-rule authority to Shiites in the oil-rich south and weaken the Sunni voice in running the country. Shiite-Sunni violence has risen dramatically since the bombing of a major Shiite shrine in northern Iraq in early 2006. The bombing is blamed on Sunnis. Shiite and Sunni extremists have clashed in Pakistan and Afghanistan, and in Lebanon relations have been tense following the assassination of former Prime Minister Rafik Hariri, a Sunni, in 2005.