But to many US lawmakers, regional experts and Middle East leaders, the chief risk is not a more menacing version of Taleban-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 drawdown and Bush opens the door to new approaches. A majority of Americans favor at least a partial withdrawal of troops, but the administration also is considering a temporary increase as part of an effort to step up training of Iraqi forces.

A study panel chaired by former secretary of state James Baker and former Democratic congressman Lee Hamilton recommended Wednesday that most combat troops be out of Iraq by early 2008.

It said the primary mission of US troops "should evolve to one of supporting the Iraqi army, which would take over primary responsibility for combat operations."

Bush has warned repeatedly of what he sees as the dangers of a swift US 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 October 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 is less risk of an al-Qaeda-like terrorist group taking control of the entire country than there is 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 Phillip Merrill Center for Strategic Studies at the Johns Hopkins University School of Advanced International Studies.

Republican senator Lindsey Graham, who favors an increase in US 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 the Iraqi state was not taken over by terrorists, he said, a US withdrawal would hearten Islamist 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."

Senator Jack Reed, one of the Democrats' point men on Iraq, sees progressive fragmentation along sectarian lines in Iraq. But he argues that a reduction in US forces would not worsen the trend "because those dynamics are going on regardless of our presence."

Even in a drawdown, Reed said, the US 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 US military commanders fear an outburst of terrorism after a withdrawal.

Army General John Abizaid, the top US commander for the Middle East, predicted in Senate testimony recently 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 official of the State Department's intelligence unit. "In the worst case, there would be only pockets of terrorists in the Sunni areas."

LOS ANGELES TIMES