During the very week when America finally got down to a real debate over Iraq, President Bush went to … Vietnam.

The symbolism is astonishing. Here was a president who avoided serving in Vietnam by joining the Texas Air National Guard, now flying to an international summit in Hanoi. And his visit followed right after a U.S. election that turned on whether Iraq has become a quagmire like Vietnam.

And there was Bush chatting up Vietnamese communists about free trade, 31 years after our exit from Saigon. Remember how that debacle sparked dire predictions that the neighboring countries would fall, like dominoes, to communism? Of course, those predictions never came true.

So I’m not surprised to be getting reader e-mail that asks whether the “doomsday predictions of what might happen if we withdraw from Iraq” aren’t exaggerated. “I remember the same was said about Vietnam if we abandoned it to the communists,” wrote one reader, “and look at our relationship now …” A fair enough point. But are the two cases really similar?

Let me start by saying Iraq is no Vietnam. The complex Iraqi situation makes the Vietnam war look simple. And I believe the consequences of a swift U.S. withdrawal would be far, far worse than the repercussions of Vietnam.

Why so? The nature of the violence in the two countries is very different. In Vietnam, the struggle was fought by a nationalist movement (with communist ideology) fighting to drive out U.S. forces.

In Iraq, the most vicious violence reflects an internal power struggle between minority Sunni Muslims who were allied with Saddam Hussein and the long-persecuted majority of Iraqi Shiite Muslims.

Hard-line members of this Sunni minority – including senior members of Saddam’s Baath party and religious fanatics – began to instigate chaos right after the invasion. They hoped to scare the Iraqi people into restoring a Sunni dictator. Sunni jihadis deliberately sought to provoke a civil war with Shiites, bombing their markets and mosques.

Had the Bush team sent more troops to Iraq for the postwar, as requested by then-Army chief of staff Eric Shinseki, the worst violence could have been prevented; most U.S. troops probably would be home now. But Shinseki was roughly rebuffed by former Defense Secretary Donald Rumsfeld. Just last week, Gen. John Abizaid, top U.S. commander in the Middle East, told the Senate Armed Services Committee: “Shinseki was right.”

Shiites refrained from revenge until last February, when the bombing of a holy shrine shattered their restraint. Iraq is now mired in a cycle of sectarian killing.

So instead of facing a coherent nationalist movement, U.S. troops are stuck in the middle of a sectarian war. The mirage of creating Iraqi democracy is shattered.

The U.S.-trained Iraqi military is divided by ethnicity and religion and has no unified government to fight for. The Iraqi police force is penetrated by Shiite religious militias and death squads. Many Iraqis already yearn for the return of a strongman.

In such circumstances it is tempting to call for a speedy U.S. exit. Some thoughtful Bush critics argue that a quick withdrawal couldn’t cause worse violence than Iraq now faces.

But unlike Vietnam, there is no strong nationalist movement in Iraq waiting to take over. That means that once a U.S. military exit is scheduled, sectarian killing will explode as Sunnis and Shiites fight to the death.

Even worse, this struggle is likely to be protracted and bloody. In Lebanon, a civil war between religious sects in the 1970s and ’80s lasted for 15 years.

Unlike Vietnam, the continuing Iraq war would be likely to spill over into much of the region, sucking in Iraq’s neighbors. Shiite Iran would help Iraqi Shiites, and Arab Sunni leaders would feel obliged to help Iraqi Sunnis. Iraq’s neighbors would fight their battles over the bloody corpse of a shattered country.

The most vicious militias, run by Sunni jihadis, would take control of chunks of Iraq. Their fiefs would serve as training fields for radical Arabs Islamists who seek to overthrow moderate rulers.

Also, unlike Vietnam – a small country in a less important region – the spillover from Iraq would affect a strategically vital region that produces oil and Islamist terrorists.

And finally, unlike Vietnam, the many Iraqis who worked with the Americans would not be sent to re-education camps, nor could they easily flee the country. Many thousands probably would be slaughtered, and their deaths would further blot America’s tattered honor.

These reasons – both strategic and moral – make me resist calling for a timetable for withdrawal from Iraq in the near term. We must first exhaust every effort to find a way to stabilize the country – and persuade Iraq’s neighbors to contribute. We owe Iraqis – and ourselves – that much.