EVEN as the bodies are pulled from the wreckage of the World Trade Centre, American gun-sights are scouring the world, looking for a target on which to exact revenge. The problem for President George W Bush and his advisors is that, while the true culprit may not be known for some time, the demand for action is immediate.

The sheer scale of the atrocity is enough by itself to provoke a swift and violent retribution. Added to this is the terrible blow to US pride, through strikes at one of the most potent symbols of western capitalism, and at the heart of American defence, the Pentagon.

"The pressure on President Bush to act will be enormous. He has got to do something," says Rod Hague, senior lecturer in politics at Newcastle University. "I guess the administration will now be entering into a debate into what that something should be.

"Ideally, they would like to nail the culprits, but that is unlikely to happen easily or quickly, and there will be a temptation to do something dramatic. I think this will crystallise some of the latent policy divisions within the Bush administration, between those who want 'Fortress America', and those who argue for a more sustained engagement in world politics.

"Both camps will use these terrible incidents to support their case, and we don't know which way the president will jump. There will be people itching to have a go, the problem is to find an appropriate target to have a go at."

But with the trails of evidence pointing to the origin of the perpetrators buried under thousands of tonnes of rubble, waiting for the right target to be identified may prove impossible. "I think it is quite possible that there will be early, and very possibly precipitate, action, and they will give in to the pressure to do something rather than wait for the results of an inquiry," says Mr Hague.

'If they have reason to believe another state is involved, I think we can assume they will be priming the cruise missiles. And if it was one of their hit-list of rogue states, they will be only to willing to press the button. But there are certainly reasons to think that presidents are capable of military action on what are very flimsy or insubstantial political grounds."

Among the Bush administration, Secretary of State Colin Powell, himself a former chairman of the joint chiefs of staff, is expected to be on the side of those urging restraint. But if his arguments do not prevail, it could lead to an escalating spiral of retribution, according to Mr Hague.

"It may be that one of the objectives of the terrorists is to provoke a cycle of violence, and that it would play into their hands. But you can't do nothing either, and you can't be sure that an investigation will produce a result."

But while the rhetoric coming from the president, and the signs of anger among the American people, suggest a retributive strike will take place, it may not be imminent, according to Pat Chilton, professor of politics and international relations at Sunderland University. "I don't think immediate retaliation is on the cards. There is going to be enormous pressure on the president to do something, and be seen to be doing something no matter what," she says. "But any immediate retaliation is out of the question, because the situation is so confused. They will have to wait a little while until things are clearer. Even the compulsion to do something big to show the US is still strong could be undermined by the damage which could be caused by precipitate action."

But when the retaliation does happen, Afghanistan is one of the countries right in the firing line, as the hiding place of the chief suspect, Saudi millionaire Osama bin Laden.

"What I fear is that, in the middle distance, there will be some kind of action that will be justified in the sense of being directed at people who harbour terrorists. President Bush has already said they are equally guilty and that prepares the ground. If they feel they have reason to believe bin Laden is responsible they could hit Afghanistan."

But, however hard the US forces strike, the difficulty is it would be almost impossible to defeat an enemy who is not confined to one country.

"It is like the failure to believe that this sort of tragedy could happen in America," argues Prof Chilton. "They still believe that, with the best military and the best intelligence, they must be able to find these people and eradicate them. "The American military knows very well that you can't eradicate this phenomenon by going out and bombing them. I can think of no way that is going to be effective.

"And it is going to be extremely difficult to avoid exacerbating the situation."

The history of US retaliation suggests it only serves to inflame hatred against the west, and provide a breeding ground for future terrorists, willing to die for their beliefs.

'There would be nothing better for bin Laden, or a collection of Palestinian and Afghanistani organisations, to have the United States go out, guns blazing, and carry out a high-profile military operation," she says. "That produces the next generation of suicide bombers, and the finance to make these sort of operations happen. It would simply make the next stage even worse.

"I don't think it would escalate into a Third World War, it is not that kind of conflict. But if it does escalate, it will build up so that, in ten years, the next atrocity could be worse. By then, the terrorists could have missile technology, or nuclear weapons. This famous shield that Bush is talking about won't even be off the drawing board then.

"There is no protection against that kind of long, slow escalation. We needn't worry about tomorrow, or next week or even next year, we're talking about the next century."

The risk of a revenge strike leading to further terrorism, and, in turn, to further retaliation, is echoed by Pandeli Glavanis, professor of Middle Eastern studies at Northumbria University. "There is bound to be some sort of military response in the Middle East and Afghanistan and I think it is going to be a much more sustained kind of war than just a few rockets," he says.

"But, unless it is done extremely carefully, it could generate a lot more opposition, and, therefore, one possibility is that it might get worse, in terms of more terrorist attacks."

Tony Blair has been quick to align himself with the US, but British involvement in a military response would also make Britain, and British citizens throughout the world, a target for terrorists, Glavanis says. And the chilling truth is, that against an almost invisible enemy, prepared to go to any lengths for their cause, there is, in the end, no protection.

"In the 21st Century, war is fought on a very different battleground," he says. "It is fought in urban environments, in civil society, which cannot be protected 24- hours a day. In the end, we have to find out and deal with what gives rise to this particular phenomenon. Until that is resolved, there will always be a risk."