Rivalries and violence between Shiite factions are threatening to overshadow progress U.S. forces have made against al-Qaida in Iraq and other extremists just weeks before the top American commander and diplomat in Iraq report to Congress.
An all-out, Shiite-on-Shiite conflict could plunge the oil-rich and mainly Shiite south of Iraq into chaos that could rival — or even surpass — the bloodshed across Baghdad and the center of the country for more than four years.
That, in turn, could shatter the relative unity in the Shiite mainstream, which has given crucial support to the U.S.-led mission in Iraq, and deepen the predicament of embattled Shiite Prime Minister Nouri al-Maliki.
Clashes between rival Shiite factions have not been uncommon since the overthrow of Saddam Hussein's regime in 2003, but the most recent ones are by far the most ominous given their deadliness, scope and timing.
For Washington, the danger of a full fledged armed conflict in southern Iraq could not have come at a worse time.
Gen. David Petraeus, America's top commander in Iraq, and Ambassador Ryan Crocker are due to report to a skeptical Congress on the Iraq war in less than two weeks.
A southern Iraq mired in political turmoil and bloodshed could only strengthen the argument of war critics and intensify calls for a drawdown on the estimated 160,000 U.S. troops now in Iraq.
Dealing with such a conflict could pose a serious dilemma for the U.S. military — to watch from the sidelines while rival factions fight it out or intervene to restore the peace.
Taking the first option would undermine the military's reputation as the ultimate guarantor of security in Iraq. The second would most certainly mean bolstering the relatively light U.S. military presence in the area — now overseen by British forces — giving the politically damaging impression of a widening war.
The major players in the power struggle in the south are the Mahdi Army militia loyal to anti-American cleric Muqtada al-Sadr and the Supreme Islamic Council of Iraq, the country's largest Shiite political party and the patron of the Badr militia.
Security forces in the region are known to be dominated by supporters of the Supreme Council, led by Abdul-Aziz al-Hakim, whose family has been locked in a long-running competition with al-Sadr's family over leadership of Iraq's Shiites.
The rivalries boiled over into deadly clashes Monday and Tuesday in the holy city of Karbala during a major Shiite religious festival. Dozen of people were killed.
In an apparent bid to deflect criticism of the Mahdi Army over the violence in Karbala, al-Sadr announced Wednesday a "freeze" of the militia of up to six months to allow for its restructuring. The move was followed by reported threats by al-Sadr aides that the feared militia would resume its activity if the arrest of Sadrist officials did not stop.
Tensions have been building up in the south in recent weeks, with two provincial governors assassinated and clashes erupting in several cities between the Mahdi Army and Badr militiamen.
The future of south Iraq looks even more uncertain because of Britain's plans for the gradual reduction of its military force in southern Iraq, where it has played a key role in keeping the peace among rival Shiite factions since its arrival in 2003.
One particularly ominous sign came Sunday when about 50 Mahdi Army militiamen systematically looted a former British-Iraqi security command.
Hassan al-Suneid, a key Shiite lawmaker from al-Maliki's Dawa party, also sought to discount the significance of the latest tensions, arguing that there has been no sign to date to suggest that irreparable damage has been inflicted on the unity of Iraq's Shiites, who account for about 60 percent of the country's estimated 27 million people and are Iraq's most dominant group.
"It is mostly a case of political jockeying," said al-Suneid. "Everyone wants to be seen as Iraq's hero and the deliverer to win a larger popular following."
The rivalry between Shiites in the south is mainly motivated by the pursuit of domination in the strategic and potentially rich Shiite heartland.
Beside the vast oil reserves, control of the south offers the wealth generated by the area's shrine cities of Najaf and Karbala, which attract millions of visitors every year and cash donations by pilgrims. The two cities also are prestigious seats of Shiite learning.
Part of the problem in the south is that the Sadrists did not take part in local elections held in January 2005, selecting to focus instead on the simultaneous parliamentary vote. That decision allowed their rivals in the Supreme Council to dominate local governments across the south, leaving the Sadrists bitter and angry over their lack of representation.
The deeper roots of the conflict may be found in the makeup of the protagonists themselves.
Al-Sadr's supporters are primarily poor Shiites who gain from the services offered by the group and obtain a sense of empowerment through membership of the Mahdi Army. In contrast, the Supreme Council is perceived as a magnet of middle- and upper-class Shiites and enjoying the endorsement of the wealthy and traditional clerical leadership.
"In many ways, the Sadrists are leading a social revolution," said Joost Hiltermann, Middle East director of the International Crisis Group, a respected research agency based in Brussels, Belgium.
"It is a struggle against the traditional political class and the wealthy merchants of the Shiite shrine cities who support the Council," Hiltermann said.
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Associated Press Writer Hamza Hendawi has covered Iraq on frequent assignments since 2003.