After publishing an op-ed entitled “The Real Islam” last summer, I was bombarded by requests to produce a follow-up piece outlining the differences between Sunni and Shi'ite Muslims. Well, here it is, with one caveat: The historical complexities and theological nuances of the 1,400-year-old rift in Islam make the 600-year division between Roman Catholicism and Protestantism, or the contemporary divisions within Protestantism, seem simple by comparison. There is no way a thousand-word essay can say it all. Thus, though I may outline these differences in a nutshell, I’m certain to leave countless other nuts yet to be cracked.

Muslims today make up about one person in four, some 1.4 billion altogether. Of these, nearly 90 percent are Sunni; the remaining 10 percent are primarily Shi'ite. Of the world’s 52 majority-Muslim states, only five are majority Shi'ite: Iran (90 percent), Azerbaijan (80 percent), Bahrain (70 percent), Iraq (66 percent) and Lebanon (50 percent). With minor exceptions, the rest are majority Sunni. These facts notwithstanding, Shi'ite Muslims exert an influence in the Muslim world and beyond that transcends their comparatively meager numbers.

As both friend and foe, Shi'ite Iraq has been a focal point of U.S. foreign policy for at least three decades. Iran’s enormous oil wealth, hard-line theocracy and pursuit of nuclear weapons continue to pose problems for the Middle East and the West. Azerbaijan’s post-Soviet dictatorship not only enjoys enormous oil and natural-gas wealth, but also functions as an unavoidable corridor for oil transport between Russia, Central Asia, Europe and the Middle East. Bahrain hosts a key U.S. naval base and enjoys a reputation as a strong and growing world financial hub. Lebanon, once the Westernized gem of the Middle East, is now plagued by Hizballah and Syrian interference but continues to hold a pivotal role regarding Israeli security and regional democratization. For better or for worse, the Shi'ites cannot be ignored.

Two prevailing issues lend urgency to our understanding of these, the two great sects of Islam. First, as Congressional Quarterly’s Jeff Stein demonstrated in a series of biting reports over the past several years, even the most senior and seasoned U.S. legislative, foreign-policy, intelligence and law-enforcement leaders have next to no understanding of the differences between Sunnis and Shi'ites, what countries are dominated by which sect, or why it matters. Second, as is so readily apparent in Iraq today, Sunnis and Shi'ites have little compunction when it comes to slaughtering each other. This is because each considers the other heretical – that is, outside the oma or community of true Islam.

Across the entire Muslim world, it’s dangerously naïve to think that the differences between Sunnism and Shi'ism are all that matter; in fact, it’s far more complicated. Yet given that these differences do matter, what are they?

It is no small detail that the rift between Sunnis and Shi'ites dates to the death of Muhammad, Islam’s founder. Shortly before his own death in 632 AD, Muhammad’s last surviving son, Ibrahim, also died. By this time, Islam was already tightly woven into a religious and political community led by a man who was at once both a religious and political leader. In the absence of an heir apparent, the question of succession – who would lead Islam after Muhammad – quickly engulfed this nascent but powerful Islamic oma.

The term Sunni comes from the Arabic word sunna, which roughly translates as “example,” indicating those who follow the example of Muhammad. Sunnis refer to themselves properly as Ahlus Sunnah wal-Jamaa'h, roughly “the people of the example [of Muhammad] and the community.” The name is meant to connote their own claim as the heirs of “orthodox” Islam and as the majority among competing Muslim sects.

The name was chosen because Sunnis believed themselves to be following the example of Muhammad in several key respects. Muhammad, they say, did not designate a successor or dictate a procedure for selecting one. Also, Muhammad’s claim to prophethood was unique – his successor would be a leader of the community, not another prophet. Finally, what was clear was that Islam should remain united under one individual – a leader of the oma, a military commander and the final arbiter of disputes within the community and interpreter of its law. Implicit in these assertions was the belief that Islam’s leader need not come from a particular family, clan or tribe.

Consequently, Abu Bakr, one of Muhammad’s inner circle and among the first converts to the new religion, became Islam’s first leader, or caliph. Serving as caliph from 632 to his death two years later, Abu Bakr was the first of whom Sunnis recognized as the four “Rightly Guided Caliphs.” Before his death, Abu Bakr named another of Muhammad’s inner circle, Umar, as his successor. Umar ruled as caliph to his death in 644, during which time he created a sort of electoral college to choose future successors. This group chose Uthman as Islam’s third caliph (644-656), followed by Ali, Muhammad’s son-in-law and cousin, who held the title until his death in 661. The caliphate continued after Ali but was marked by increasing political disunity and corruption through several dynasties, causing Muslims to look back on the era of its first four caliphs as the “Golden Age” of Islam.

Golden to the Sunnis, that is. The Shi'ite minority, by contrast, considered Ali as the rightful heir of Islam, designated as such by Muhammad himself. The intervening three leaders – Abu Bakr, Umar and Uthman – were, therefore, illegitimate. They stole Ali’s position from him, Shi'ites say, despite all three having been present when Muhammad allegedly designated Ali as his successor at the oasis of Ghadir al-Khumm.

For the Shi'ites, Ali was no mere caliph; to them, he enjoyed a similar – but not identical – prophetic status as Muhammad before him. Whereas Muhammad received revelation from Allah (wayh), Ali and his successors received divine inspiration (ilham) allowing them to guide and judge Islam sinlessly, both spiritually and politically. Thus, for Shi'ites – also called Shi'a Ali, the “party of Ali” – Ali is the first imam, the leader of the oma descended from Muhammad. To them, the Imamate, not the Caliphate, is the rightful ruler of Islam.

In 874, Muhammad al-Qa'im became the twelfth imam at the age of six, and the end of Muhammad’s line. Shi'ites claim that for the next 67 years he existed in a state of “lesser occultation,” where he was directly accessible to his followers, followed by an inaccessible “greater occultation” which will continue until the Last Days. When this “Hidden Imam” is again revealed, he will initiate an apocalyptic struggle against the foes of Islam, hailing the end of the world. In the meantime, the rule of Islam resides in the ayatollahs, the “sign of Allah,” who act in the name of the Hidden Imam.

In the lead up to the 1979 Iranian revolution, the Ayatollah Khomeini never directly claimed to be this Hidden Imam, but his followers propagated the idea in order to legitimize Khomeini’s claim against the secular government of the pro-Western Mohammad Reza Pahlavi, Shah of Iran, and to consolidate power after the Shah’s exile. Today, Iranian President Mahmud Ahmadi-Nejad purports to be making preparations for the Imam’s second coming, which he believes to be close at hand. While Ahmadi-Nejad is doubtless a true believer, this claim may be intended to buttress the hard-line theocracy’s often-tenuous rule in Iran, as well as helping mobilize fellow Shi'ites across the border in Iraq.

There, Shi'ite-Sunni relations will be at the heart of conciliation or disintegration – whichever may occur. Indeed, religious, ethnic and tribal divides define four conflicts being simultaneously waged in Iraq today: Shi'ite on Shi'ite violence in the south; Shi'ite-Sunni sectarian violence in Baghdad; Ba'athist-inspired violence against the government; and al-Qa'ida/jihadist violence against anti-Western Shi'ites and the pro-U.S., Shi'ite-dominated government.

While religious violence is not the exclusive cause of the violence in Iraq, without it the conflict would be greatly simplified and far more manageable. As it is, working to quell one of Iraq’s conflicts often has the result of inflaming another.

It may be an oversimplification to say so, but a Vatican II-styled resolution between Sunnis and Shi'ites may be just what the Muslim world – and the rest of the world – needs most.