Hadi Mizban, AP
Shiite Muslim worshippers pray at the holy shrine of Imam Abbas during the annual festival of Shabaniyah, which marks the anniversary of the birth of the ninth-century Shiite leader known as the Hidden Imam, in Karbala, 50 miles (80 kilometers) south of Baghdad, Iraq, Monday, June 24, 2013.

The news is full of stories of Sunni Muslims and Shi‘ite Muslims, and, too often, of strife between them. Yet many in America probably don’t fully understand the difference between these two main groups within the Islamic world.

When the Prophet Muhammad died, rather unexpectedly, in AD 632, there were two major responses among the believers. (We’re oversimplifying, but not, we think, too much.) No significant group believed that Muhammad would be succeeded by another prophet; revelation, all agreed, ended with the completion of the Quran. But a ruler was needed in order to preserve Islamic law, collect offerings and supervise worship.

One group said that, while someone needed to succeed him as political leader — at his death, Muhammad ruled most of the Arabian Peninsula (roughly modern-day Saudi Arabia, Yemen, Oman and the Gulf States) — it wasn’t especially important who that person was. They eventually came to be known as “Sunnis,” from the Arabic term “sunna,” meaning “tradition” or “customary practice” (of the Prophet). Calling oneself “Sunni” is, in effect, to label oneself “orthodox,” as in “We follow the sunna of the Prophet, unlike, well, you.”

The other group, not unlike the “Reorganized” LDS Church that emerged some years after the death of Joseph Smith, insisted that it was vitally important that the Prophet be succeeded by his nearest male relative. Unfortunately, Muhammad had no son who lived to maturity. So they coalesced around his nearest adult male relative, his adopted brother and son-in-law, Ali, who was also his cousin and a hero of the early Islamic wars. Ultimately, this group came to be called the “shi‘at Ali,” or “faction/supporters of Ali.” Hence the title “Shi‘ite.”

But Ali was young, and three older men became “caliphs” or successors, assuming the leadership of the Islamic empire, before Ali came to power. And, even then, he ruled for only five years before being assassinated. Thereafter, as the years passed and his family remained out of power, Shi‘ite allegiance passed from father to son to grandson. However, as Ali’s descendants multiplied, disagreements arose as to which line of his posterity was the rightful claimant.

Plainly, the original difference between the two tendencies was essentially political. But it gradually extended to doctrinal and liturgical matters. Sunni scholars, seeking precedents for Islamic law and guidance on the interpretation of the Quran, turned to the so-called “companions” or closest supporters of the Prophet for guidance. Shi‘ites, by contrast, regarded those “companions” as the very people who had unjustly excluded Ali and his heirs from the caliphate. So they sought their guidance from whichever line of descendants of Ali — known as imams (“leaders”), they recognized. It was inevitable that, over time, interpretations would diverge.

One important Shi‘ite schism occurred at the death of the seventh imam; his followers are thus sometimes called “Seveners,” they’re more often known under the title of “Isma‘ili” Shi‘ism. They have a major modern presence in parts of India and in East Africa, and have created a rich though sometimes heavily guarded legal and philosophical literature.

By far the largest Shi‘ite group is the “Twelvers” or the “Imami” Shi‘ites. Today they rule Iran, claim the majority of Iraqis, and form a large minority in the south of Lebanon. (The militant faction Hezbollah is a Twelver Shi‘ite organization.) Their name derives from their allegiance to the “Twelfth Imam,” Muhammad al-Mahdi. They believe that when, after considerable persecution, the eleventh imam died (perhaps by poisoning) in AD 874, he left behind him a little son who, out of concern for his safety, was hidden away. He lives still as a sort of “translated” man and will someday emerge from hiding to establish Islamic rule and, thereby, bring justice and peace to the world. Bashar al-Assad, the dictator of Syria, belongs to a quasi-Shi‘ite “Alid” sect, adding an extra sectarian dimension to the terrible civil war there.

Within Islam globally, Shi‘ites constitute a relatively small minority of 10-13 percent — which still yields somewhere between 150 and 200 million Muslims, or perhaps even somewhat more. In the heartland of the Middle East, between Egypt and the Levant on the west and Iraq and Iran on the east, the numbers of Shi‘ites and Sunnis approach much closer to parity. Thus, with an assertive and powerful Shi‘ite state in Iran and with the replacement, in Iraq, of Saddam Hussein’s minority Sunni dictatorship by a more democratic government that reflects that country’s Shi‘ite majority, the stage is set for ongoing sectarian conflict.

William Hamblin is a professor of history at BYU, where Daniel Peterson teaches Islamic studies and Arabic They write extensively on ancient, medieval and Mormon subjects, including regular blogs. Their views aren't necessarily those of BYU.