The majority of the world's Muslim population follows the Sunni branch of Islam, and approximately 10%-15% of all Muslims follow the Shiite (Shi'ite, Shi'a, Shia) branch. Shiite populations constitute a majority in Iran, Iraq, Bahrain, and Azerbaijan. Sunnis and Shiites share most basic religious tenets. However, their differences sometimes have been the basis for religious intolerance, political infighting, and sectarian violence. This report includes a historical background of the Sunni-Shiite split and discusses the differences in religious beliefs and practices between and within each Islamic sect as well as their similarities. The report also relates Sunni and Shiite religious beliefs to discussions of terrorism and sectarian violence that may be of interest to Congress.

This report discusses Iran’s influence over the post-Saddam government in Iraq, which is substantial because the predominant parties in that government have long enjoyed Tehran’s sponsorship. An emerging concern is that Iran’s influence has extended to support for militant groups in Iraq. Some U.S. statements and press sources say that sophisticated explosive devices are entering Iraq from Iran, suggesting that Iran, or factions within Iran, are backing Iraqi factions that use violence to oppose the U.S. and allied military presence in Iraq.

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