“The Sunni and Shiite sects of Islam encompass a wide spectrum of doctrine, opinion and schools of thought. The branches are in agreement on many aspects of Islam, but there are considerable disagreements within each.”

Saudi Arabia’s execution of the Shiite cleric Sheikh Nimr al-Nimr could escalate tensions in the Muslim world even further. In the Shiite theocracy Iran, the supreme leader, Ayatollah Ali Khamenei, said on Sunday that Saudi Arabia, which is ruled by a Sunni monarchy, would face “divine vengeance” for the killing of the outspoken cleric, which was part of a mass execution of 47 men. Sheikh Nimr had advocated for greater political rights for Shiites in Saudi Arabia and surrounding countries. Saudi Arabia had accused him of inciting violence against the state.

Here is a primer on the basic differences between Sunni and Shia Islam.

What caused the split?

A schism emerged after the death of the Prophet Muhammad in 632, and disputes arose over who should shepherd the new and rapidly growing faith.

Some believed that a new leader should be chosen by consensus; others thought that only the prophet’s descendants should become caliph. The title passed to a trusted aide, Abu Bakr, though some thought it should have gone to Ali, the prophet’s cousin and son-in-law. Ali eventually did become caliph after Abu Bakr’s two successors were assassinated.

After Ali also was assassinated, with a poison-laced sword at the mosque in Kufa, in what is now Iraq, his sons Hasan and then Hussein claimed the title. But Hussein and many of his relatives were massacred in Karbala, Iraq, in 680. His martyrdom became a central tenet to those who believed that Ali should have succeeded the prophet. (It is mourned every year during the month of Muharram.) The followers became known as Shiites, a contraction of the phrase Shiat Ali, or followers of Ali.

The Sunnis, however, regard the first three caliphs before Ali as rightly guided and themselves as the true adherents to the Sunnah, or the prophet’s tradition. Sunni rulers embarked on sweeping conquests that extended the caliphate into North Africa and Europe. The last caliphate ended with the fall of the Ottoman Empire after World War I.

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