With Americans leaning consistently in favor of disengagement from Iraq, President Bush has warned that a precipitate withdrawal would create a terrorism superstate in the Middle East that is rich with oil cash and determined to topple moderate governments around it.
But to many U.S. lawmakers, regional experts and Middle East leaders, the chief risk is not a more menacing version of Taliban-dominated Afghanistan, but a Lebanese-style civil war that could result in the deaths of thousands more Iraqis and expand the conflict by drawing in neighboring states.
The sharply differing views color the growing debate over the consequences of withdrawal as incoming Democratic congressional leaders demand a troop drawdown and Bush opens the door to new approaches. A majority of Americans favor at least a partial withdrawal, but the administration also is considering a temporary troop increase as part of an effort to step up training of Iraqi forces.
A study panel chaired by former Secretary of State James A. Baker III and former Rep. Lee Hamilton (D-Ind.) will offer recommendations today for gradual troop withdrawals. Its final report is expected to reject a quick withdrawal, but key members have signaled that the U.S. commitment eventually should come to an end.
Bush has warned repeatedly of what he sees as the dangers of a swift exit.
"If we do not defeat the terrorists and extremists in Iraq, they will gain access to vast oil reserves and use Iraq as a base to overthrow moderate governments across the broader Middle East," he said Oct. 25 at a White House news conference.
Terrorists "will launch new attacks on America from this safe haven," Bush said, and "will pursue their goal of a radical Islamic empire that stretches from Spain to Indonesia."
But many others believe the increased risk of terrorism would be confined to limited portions of Iraq.
Some experts argue that many Iraqis, including some Sunni Arabs, as well as Shiites and Kurds, already are unhappy with foreign terrorists and would try to drive them out.
John McLaughlin, former acting CIA director, said there was less risk of an Al Qaeda-like terrorist group taking control of the entire country than of a "civil war in which you have the disintegration of the country and a widening set of tensions and potential conflicts throughout the Middle East."
In such circumstances, "Al Qaeda could gain a foothold in some piece of territory," said McLaughlin, now at the Philip Merrill Center for Strategic Studies at the Johns Hopkins University School of Advanced International Studies.
Sen. Lindsey Graham (R-S.C.), who favors an increase in U.S. troops, agreed that the greatest risk is not an Al Qaeda takeover but a "wider regional war around ethnic and religious differences."
But even if Iraq wasn't taken over by terrorists, he said, a U.S. withdrawal would hearten Islamic militants and strengthen them in their fight against moderate states in the region.
Neighboring countries may initially choose a side and support it with money and weapons. Sunni-led states, perhaps Syria and Saudi Arabia, could come to the aid of Sunni groups, while Iran might help the Shiite south, said Stephen Biddle, a military affairs specialist at the Council on Foreign Relations.
But if it appears that their allies are about to lose, the states might feel pressure to increase their commitment, and might escalate the conflict by sending in their own troops, he said.
"I see a lot of warfare going on here before anyone can form a stable state out of Iraq," Biddle said. "We're more likely to get a regional version of Lebanon, and chaos, than the quick formation of a new order under a party we don't like."
Sen. Jack Reed (D-R.I.), one of the Democrats' point men on Iraq, sees progressive fragmentation along sectarian lines in Iraq. But he argues that a reduction in U.S. forces would not worsen the trend, "because those dynamics are going on regardless of our presence."
Even in a drawdown, Reed said, the U.S. could train and support Iraqis, and could continue to go after Islamic militants in such places as Sunni-dominated Al Anbar province.
Syrian President Bashar Assad, among other Middle Eastern leaders, has said that the growing sectarian pressures in Iraq are likely to draw in other countries.
"Almost all countries have breaking points, and when the ethnic-religious break occurs in one country, it will not fail to occur elsewhere too," Assad recently told the German magazine Der Spiegel. "Large wars, small wars -- no one will be able to get a grip on the consequences."
Like Bush, some U.S. military commanders fear an outburst of terrorism after a withdrawal.
Army Gen. John P. Abizaid, the top U.S. commander for the Middle East, predicted in Senate testimony this fall that the Sunni Arab areas of central and western Iraq would not be viable as an independent state and would turn into a lawless haven for groups such as Al Qaeda.
But other analysts believe Sunnis would have little tolerance.
"We know they're already rather sick of the foreign terrorists, and could boot them out," said Wayne White, a former State Department intelligence official. "In the worst case, there would be only pockets of terrorists in the Sunni areas."
A U.S. withdrawal also would affect the growing regional influence of Iran, but there is little agreement on what a pullout would mean for Tehran.
Some, such as Graham, believe that "Iran would be the biggest winner" of a U.S. exit because it would gain strong influence over Iraq's Shiite south and an increased ability to dominate others in its region.
But others believe that though the U.S.-led invasion of Iraq has greatly increased Iran's regional influence, a withdrawal would be a setback, forcing it to become a caretaker of disorderly southern Iraq and threatening to embroil Tehran in a costly regional civil war.
"The American presence has greatly facilitated an expansion of Iranian influence, and they would probably prefer to have us stay and continue to bear the burdens," said James Dobbins, a veteran diplomat who was a senior envoy for the Bush administration and is now director of Rand Corp.'s International Security and Defense Policy Center.
Indeed, some analysts believe that a withdrawal could benefit the United States by breaking up the alliance between Iran and Syria. That could happen, they say, should Syria side with the Sunni west as Iran comes to the aid of the Shiite south.
A civil war could reduce Iraq's oil output and increase world prices, but most experts doubt that Iraqi reserves would be used as a weapon against the U.S., even if Iraq comes under the influence of Iran.
"They're going to need the money," said Peter Khalil, an analyst at Eurasia Group, a risk-advisory consulting firm, and a former advisor to the Coalition Provisional Authority in Iraq. "They're not going to want to shoot themselves in the foot."