Islam's Sunni-Shiite split

Nevertheless, there are some key religious differences. Shiite veneration of the holy family, that is, the descendants of Muhammad, has contributed to a much more centralized and hierarchical clergy than in the Sunni world.

All religious Shiites nominally observe the advice of an ayatollah on how to follow the law of Islam, or sharia, in the modern context. For many in Iraq, this role is fulfilled by Ayatollah Ali al-Sistani.

Sunni Islam is much less centralized. In this respect, the differences between Sunni and Shiite Islam superficially approach the differences between the Roman Catholic Church and most Protestant denominations.

Though a majority in Iran and Iraq, Shiites make up just 15 percent of the world's Muslims. Their history of defeat and frequent subjugation has also led to a cult of death and martyrdom within Shiism.

The major Shiite holidays celebrate the glorious defeats and martyrdoms of Imam Ali and Imam Hussein, Ali's son, as typified by the preeminent Shiite holiday of Ashura, which marks the slaughter of Hussein and his followers outside the Iraqi city of Karbala by a Sunni caliph in 680.

In Iraq and Iran, the holiday is marked by elaborate processions of men reenacting their own passion play, many of whom self-flagellate with chains to the beat of drums.

Such expressions of piety are looked at with disgust by hard-line Sunnis like the clergy in Saudi Arabia, who view the veneration of Hussein and other members of the prophet's family as a violation of monotheism. This view has frequently led extremist groups like Al Qaeda to attack Shiites as heretics.

The fact that Shiites have long been oppressed – first under the Ottoman Empire, later under states like Iraq and Saudi Arabia – has led to a strong identification with the injustices suffered by Hussein, and have lent a political dimension to Shiite worship. Ashura celebrations, for instance, were banned under Saddam Hussein, who feared they could lead to spontaneous uprisings.

One of the most important distinctions between Shiite and Sunni belief is veneration of the imams.

Most Shiites believe that there were 12 legitimate successors to Muhammad as caliph, and that the final imam, now called the Mahdi, disappeared when he was taken up in the arms of God. Many Shiites believe the Mahdi will return to earth one day and play the role of savior. A battle between the forces of good and evil will ensue, ending in a thousand-year reign of peace and the end of the world.

In practice, this leads to occasionally apocalyptic rhetoric from leaders like Iraq's Moqtada al-Sadr and Iranian President Mahmoud Ahmedinejad.

• Staff writer Matt Bradley contributed reporting.

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