As U.S. troops take control of Iraq, one of the great unknowns is the reception they will get from the country's Shiite Muslims, the long-oppressed majority of Iraq's 24 million people. The Shiites hold the key to stability in a post-Saddam Hussein Iraq.

If liberated from repression and assured that their interests will be served by a new government, the Shiite Muslims will probably wish to remain citizens of a unified Iraq. If they aren't liberated and reassured, they could seek autonomy or even independence -- leading to the breakup of Iraq -- or they could align themselves with their co-religionists in Iran, where Shiite Islam is the official religion. That outcome would cause deep anxiety in Saudi Arabia, which has a restive Shiite minority in its Eastern Province, where the oil fields are.

Although the Shiites represent an estimated 55 percent to 65 percent of the population in Iraq, which has long been a center of Shiite scholarship and pilgrimage, they have been politically marginalized by Hussein's regime, which is dominated by the other branch of Islam, Sunni Muslims. Anticipating American support, the Shiites of southern Iraq rose against the Baghdad government during the 1991 Persian Gulf War. When no help came, their rebellion was brutally crushed by the Iraqi military.

According to the State Department's most recent annual report on religious freedom abroad, the Iraqi government "for decades has conducted a brutal campaign of killings, summary execution, arbitrary arrest, and protracted detention against the religious leaders and followers of the majority Shi'a [Shiite] Muslim population. . . . The regime systematically has killed senior Shi'a clerics, desecrated Shi'a mosques and holy sites, interfered with Shi'a religious education, and prevented Shi'a adherents from performing their religious rites."

Yitzhak Nakash, a professor at Brandeis University and a specialist in the Shiites of Iraq, wrote earlier this month that the Shiites' preferred outcome is a unitary Iraqi state in which their heritage as Arabs would be protected, rather than an alliance with Persian, non-Arab Iran. Shiite troops constituted the majority of the Iraqi infantry during Iraq's eight-year war with Iran in the 1980s, Nakash noted in a paper for the Washington Institute of Near East Policy. They were also alienated from Iran when Tehran's religious government failed to support them during their abortive 1991 rebellion.

"If Iraq were divided into separate statelets following a war, the Shi'is would likely lose Baghdad (where they constitute nearly half the population) . . . and any share of the revenues from northern oil wells. Given that they already compose the core of the country's middle class and secular intelligentsia, the Shi'is would much prefer to seek power within a unified postwar Iraq," Nakash wrote. "Moreover, the states that would emerge from a divided Iraq would be too weak to influence regional affairs, whereas a united Iraq might allow Iraqi Shi'is to become a strong regional voice."

The schism between Sunni and Shiite Muslims erupted in the first few decades after the religion was founded by the Prophet Muhammad in the seventh century. The principal cause was not a disagreement over doctrine, but rather a dispute over succession to Muhammad's temporal authority over the Muslim state he created.

The prophet left no male heir when he died in 632. No one could claim succession to Muhammad's position as spiritual leader and revelator of the Koran, but someone had to lead the community of believers. A small group of the prophet's closest companions selected Abu Bakr, an early convert to Islam and father of one of Muhammad's wives, as caliph, or successor. They passed over Ali, Muhammad's cousin and son-in-law. That decision was at the root of the schism that still divides Islam.

The name Shiite derives from Shi'at Ali, or partisans of Ali. They believe that the true mantle of leadership can pass only to direct descendants of the prophet -- that is, to the line that began with Ali and his wife Fatima, daughter of Muhammad. The other branch of Islam, known as Sunni, accepted selection of the caliph by consensus of the community. Sunnis represent 85 percent of the world's 1.2 billion Muslims and are the majority by far in all Muslim countries except Iraq and Iran, where 87 percent of its 66 million people are Shiite.

Abu Bakr died after two years as caliph. He was succeeded by Umar, another early convert. Third in line was Uthman, a Meccan aristocrat. Only after Uthman was assassinated in 656 did Ali become caliph. His selection set off a civil war that further split the Muslim community.

Ali was challenged by companions of the prophet from the group that had orchestrated the original decision to bypass him. The prophet's widow, Aisha, supported these dissidents. Ali's troops triumphed in a bloody engagement known as the Battle of the Camel. Ali thus acquired the stigma of having been the first caliph to lead Muslims into battle against other Muslims, in contravention of the Islamic ideal of brotherhood.

Then, challenged for the caliphate by Muawiyah, the governor of Syria, Ali made a fatal political blunder: He agreed to submit to arbitration. This set off a rebellion by believers who said it was wrong to put in human hands an issue that had been decided by Allah. Worse yet for Ali, the arbitrators sided with Muawiyah, who moved the seat of the caliphate to Damascus. Ali lived on as a powerless pretender until he was murdered in 661. The Shiites have never reconciled themselves to this outcome.

Ali's tomb is in Najaf, Iraq, a center of Shiite Muslim pilgrimage and worship. To Shiites, the Holy Shrine of Imam Ali is the third holiest site in Islam -- after Mecca and Medina -- and is off-limits to nonbelievers. Ali's son Hussein, killed by troops loyal to the Sunni caliph in 680, and revered by the Shiites as a martyr, is buried in Karbala, Iraq, another holy city where only the Muslim faithful may enter his shrine.

In the uprising of 1991, Karbala was the epicenter of an insurrection that became a bigger threat to Saddam Hussein's government than the war that had just ended. Thousands of Shiite Muslim rebels commandeered two gold-domed mosques and lynched scores of government loyalists before the army retook the city.

According to Dilip Hiro's Dictionary of the Middle East, Shiite Muslims differ from Sunnis in "doctrine, ritual, law, theology and religious organization." Both groups, however, are monotheists who accept the prophecy and teachings of Muhammad and the sanctity of the Koran. Shiites and Sunnis appear indistinguishable when making the annual pilgrimage to Mecca, Saudi Arabia.

No one holds or claims the position of caliph today. The caliphate was abolished in 1924 by the secular government in Turkey after the demise of the Ottoman Empire.

In 1998, Iraqi President Saddam Hussein, a Sunni Muslim, visited the Holy Shrine of Imam Ali in the town of Najaf, south of Baghdad, in an effort to gain support from Shiites who worship there. Iranians pray at the al-Kadhemeiya mosque last December in Baghdad, which regulates the religious tourism of Iranian Shiites who want to visit holy shrines in Iraq.