WASHINGTON – If the revamped Iraq war plan fails, it will be time to withdraw most U.S. troops. Or send more in.

The United States is seen as having a limited number of options, all grim, if President Bush’s “new way forward” hits a wall. The pressure for U.S. disengagement will be immense. Yet a further escalation, however unimaginable now, may not be out of the question.

Few expect helicopters to beat the air over Baghdad in a hasty retreat of the kind that closed the books on America’s defeat in Vietnam. The Mideast and its oil are too important.

“We were able to walk away from Vietnam,” said Arizona Republican Sen. John McCain, who was a prisoner of war there. “If we walk away on Iraq, we’ll be back, possibly in the context of a wider war in the world’s most volatile region.”

The administration is almost certainly considering fallback options if the latest plan falls apart. Officials are loath to talk about them.

Over two days of intense hearings on Capitol Hill last week, lawmakers raised questions at the margins about a Plan B even as they probed and for the most part attacked Plan A: Bush’s move to increase U.S. forces to give Iraqis more time to take control of their own security.

Administration officials, defending a war that many in the U.S. and much of the world thinks is failing already, would not discuss what options will be left if the new approach fails. “Re-evaluate our strategy,” Defense Secretary Robert Gates said.

Yet he hinted that the planned troop increase need not take place in its entirety if conditions change. “We are trying to construct this in a way that there are off-ramps,” he said, so that “you don’t necessarily have to go to the full extent of the buildup.”

Gen. Peter Pace, chairman of the Joint Chiefs of Staff, testified, “You’re asking a Marine who’s focused on winning whether he has a plan in case he doesn’t win.”

“Yes,” came the response from Rep. Kirsten Gillibrand, D-N.Y.

Pace would not address the question. Nor would he discuss any exit strategy – a “pejorative get-out-of-town” phrase he said he did not like.

But if the plan does not work, he said, “our flow of forces will allow us to modify what we do next.”

Bush’s plan to add 21,500 more U.S. troops to the 132,000 already in Iraq will add to the stress on a military already overburdened and force longer and more frequent tours of duty on combat troops. An additional escalation would intensify the problem – with no ready solutions on how it would be achieved.

Those who favor Bush’s plan and many who oppose it are in striking agreement the consequences will be dire if it flops. Gates echoed the Iraq Study Group in outlining some of the likely results of failure in Iraq. He mentioned:

• Undermined U.S. credibility.

• Risk of a regional conflagration.

• An emboldened Iran.

• A “humiliating” defeat against extremism worldwide.

• A haven for terrorist networks in the heart of the Middle East.

“Should we withdraw prematurely, we could well leave chaos and the disintegration of Iraq behind us,” Gates said.

Just as success in the fight against terrorism cannot be marked by a surrender on a battleship – as the administration likes to recall, the way Japan ended World War II – failure cannot be neatly defined in Iraq. Bush set benchmarks, but no timetable for the Iraqi government to meet them. Violence on the street ranges between terrible and worse.

But if a failure that no one can deny comes about, what then?

“I don’t know what the administration does at that point,” said Ted Galen Carpenter of the libertarian Cato Institute. Bush could propose another escalation or just pull out and take the hit on U.S. credibility, he said.

“This is the foreign policy equivalent of having invested in the stock of Worldcom or Enron,” Carpenter said. “The longer you stay, the worse it gets. It’s not easy to withdraw. It’s just better than staying.”

There is no shortage of suggestions from people who think they have a better idea. Among them are an Iraqi version of the Depression-era Civilian Conservation Corps; mobile fingerprinting devices to track the worst thugs on the street; accelerated reopening of abandoned factories; a greater U.S. effort against al-Qaida in Anbar province, less against the sectarian warriors.

Kevin Ryan, a retired Army brigadier general who specialized in strategy and now is at Harvard, said some U.S. troops should be engaged in training and security no matter what becomes of the bulk of the force.

He said the U.S. should be reducing forces now and settling the remaining troops in for a long haul. “We are sprinting when we should be running a long-distance race,” he said, predicting there will be no further escalation.

“Any further surges would just cause more damage to our ground forces’ readiness,” he said. “Even if you could magically create trained soldiers … you don’t have the equipment.”

Failure of the plan also would intensify pressure on the administration to enlist countries of the Middle East in what is left of Iraq’s security. Bush might have to bend on his refusal to deal directly with Iran and Syria, but from a position of weakness.