Though the two main sects within Islam, Sunni and Shia, agree on most of the fundamental beliefs and practices of Islam, a bitter split between the two goes back some 14 centuries. The divide originated with a dispute over who should succeed the Prophet Muhammad as leader of the Islamic faith he introduced.
Today, about 85 percent of the approximately 1.6 billion Muslims around the world are Sunni, while 15 percent are Shia, according to an estimate by the Council on Foreign Relations. While Shia represent the majority of the population in Iran, Iraq, Bahrain and Azerbaijan, and a plurality in Lebanon, Sunnis are the majority in more than 40 other countries, from Morocco to Indonesia.
Despite their differences, Sunni and Shia have lived alongside each other in relative peace for most of history. But starting in the late 20th century, the schism deepened, exploding into violence in many parts of the Middle East as extreme brands of Sunni and Shia Islam battle for both religous and political supremacy.