Mark LeVine, an expert in Middle Eastern history and policy and a frequent contributor to Beliefnet, explains the historical schism in Islam and its repercussions today.

Why is the Golden Mosque of the Askariya Shrine in Samarra, which was bombed yesterday by Sunni insurgents, so holy among Shi'a?
The mosque itself was built in many stages. It was first constructed in 944 CE and was reconstructed numerous times over the centuries. But the "golden dome" that has made it famous was built in 1905. The fact that this shrine is located in the predominantly Shiite city of Samarra reminds us of how intertwined Sunni and Shi'a landscapes are in Iraq. Indeed, when I was in Iraq in 2004 one of the most common refrains I heard in meetings with numerous religious leaders on both sides was the strong desire to avert exactly the kind of sectarian war that seems to be in the making today.
The Askariya Shrine is important because it is the burial place of two of the 12 imams considered to be the rightful successors by Shiites to the fourth Caliph, Ali; specifically the tenth and eleventh imams, Ali al-Hadi and his son, Hassan al-Askari. Increasing the importance of this location is the belief among Shiites that the twelfth, or final imam, was last seen at the shrine before he disappeared.
So attacking this shrine is like attacking the very heart of Shi'a history and theology and reminds Shiites of the intense persecution they've suffered at the hands of Sunnis across Islamic history.
Moreover, because of the deeply spiritual and even mystical attributes these imams have for Shiites, it's hard to imagine a more provocative act than this bombing, short of attacking the shrines of Ali or of Hussein and Hassan. The shrine is a major pilgrimage site for Shiites as well, and aside from the religious complications, its loss for the time being will further hurt the economy of the Samarra region.
What are the main differences between Shi'a and Sunni Muslims?
In order to answer this question we need to understand the history that produced the conflict today. The current sectarian violence in Iraq has both a long history and a very short one. From the perspective of Islamic history, its roots lie in the debates over who had the right to succeed the Prophet Muhammad as the legitimate leader, or "caliph," of the burgeoning Muslim community. Shiism as a distinct sect within Islam began when followers, or "partisans" (Shi'at in Arabic) of the Prophet's cousin and son-in-law, Ali, believed that he was wrongfully passed over as Caliph in favor of Abu Bakr, Umar and Uthman, the first three leaders after Muhammad. Abu Bakr and Umar favored the early companions of the Prophet, and the "ansar," or helpers of the Muslim community during its hijrah (flight) or period of flight to Medina. Uthman, however, was from the powerful Umaya clan of Muhammad's Quraysh tribe, and when he assumed power he changed this policy toward one favoring the pre-Islamic aristocracy of Mecca.
This policy eventually led to Uthman's assassination and the appointment of Ali as Caliph. While well known for his closeness to the Prophet and personal piety (he was among the first converts to Islam) his coming to power seemingly with the help of Uthman's assassins compromised his position. The civil war that gradually engulfed the Islamic community was among the most important events in Muslim history since what started as a political battle ultimately split Islam into two theological camps, Sunnis and Shi'a.
Ali was eventually killed by a member of the Khariji faction (the most extreme faction of Ali's supporters, who turned against him when he agreed to arbitration over the conflict) and the majority of the Muslim community backed one of his opponents, Mu'awiya, who was from the same Umaya clan as Uthman, as Caliph. A smaller faction, called the "shi'at ali," or partisans of Ali, backed the assumption of Ali's sons to the Caliphate on the belief that the office of Caliph should remain only within the family of the Prophet and their direct descendants, which Mu'awiya was not.
Mu'awiya ruled from 661 till 680; after his death civil war broke out again under the reign of his son Yazid (680-83). The battle that would determine the fate of Islam took place in Karbala, in what today is Iraq, in 680. In this battle Ali's son Hussein and dozens of his followers were ambushed and killed by Yazid and his forces.
The massacre became the seminal event in Shi'a belief, as members of the sect bestowed a mythical importance to his martyrdom, which came to mirror their own suffering across Muslim history under Sunni rule. Because of the belief among Shiis that only Ali's heirs should be Caliph, a succession of twelve "imams" (leaders, or guides) were recognized as the rightful successors to Hussein as the legitimate leaders of the Muslim community.
Not surprisingly, the entrenched Umayad Caliphate that was founded by Mu'awiya had by this time moved its base to Damascus and refused to recognize the Shiite imams as caliph. Each was forced to live more or less in hiding or under government supervision, until the final Imam, Muhammad al-Mahdi, disappeared in the mid-tenth century. Shiites believe that the final imam will one day return from hiding (or in Shi'a terminology, the "Greater Occultation") as the Mahdi, or savior, and punish the sinful, reward the good, and separate truth from falsehood.
While both Sunnis and Shiites uphold the five pillars of Islamic faith, Shiites have put much greater focus on the imams as personal exemplars of Islamic piety and belief, and have attributed to them the kinds of mystical powers that orthodox Sunni Islam has refused to grant to its leaders (although the Sufi tradition of "sainthood" serves a similar function). As we can see, then, the schism between Sunnis and Shiites, in which politics, economic, and theological differences have always meshed together, started in Iraq, so it is no surprise that there is continued conflict to this day.
However, particularly in Iraq, Sunnis and Shiites have not always been at war with each other. While Shiites have periodically been persecuted in the modern period, particularly under the rule of Ottoman Sultan Abdulhamid (1876-1909), on a personal and communal level Sunnis and Shiites have long intermarried. Indeed, it is quite common to hear Iraqis proudly describe their families as having members of both sects, or being "all mixed up," as one friend described his family to me. And during the Iran-Iraq war most Iraqi Shi'a fought for Saddam Hussein, choosing their national identity over their sectarian one.
But the repression of Shiites by Hussein, which had more to do with power politics (his power base was among Sunni Arabs, particularly from his home town of Tikrit) than any religious convictions on his part, created a system of relative privilege for the minority Sunni population that Shiites have naturally wanted to end now that they have assumed power as Iraq's largest community. The two groups could have worked out some kind of arrangement but for the fateful decision of conservative Sunni leaders to tolerate and even encourage the entrance of foreign Sunni fighters, or "jihadis" into Iraq as part of their insurgency against the U.S. and coalition forces.
While the Iraqi Sunni leadership clearly thought that it could control the foreign fighters and use them to expel the U.
S., the fighters, epitomized by Musab al-Zarqawi and his al-Qaeda offshoot, had a very different agenda, which was to attack the foundation of Shiite power in Iraq as part of a larger war against Shiites, which in the minds of the extremist "Salafis," or ultra-orthodox Sunni Muslims, are "worse than Jews and Crusaders" because of what in their minds is the traitorous history and heretical theology of Shiism.
Yet if there is anything positive that is coming out of this tragedy it is the increasing willingness of Sunnis to come out forcefully against the actions of Zarqawi and other jihadis, who have focused on attacking Shiites as a primary goal. Thus in the streets of Iraq today, and across the Muslim world, Sunnis have blamed the "excommunicators" for these crimes, thereby labeling Zarqawi and those like him as apostates for their actions.