David Clark: Mr Blair must be prepared to stand up to President Bush

'If the US is seen to be settling old scores it will increase resentment against the West'

MPs returning to the House of Commons today to debate the political and military consequences of Tuesday's mass carnage in New York and Washington have a difficult and important job. Often mocked for its irrelevance to the day-to-day workings of government, it is in times of grave security crises that the Commons comes into its own as the voice of national opinion. In no other context are the actions of the executive so conditioned by the mood of the legislature. The difficult part will be finding the right balance between competing emotions and demands.

On the one hand, there will be a desire to express solidarity with the United States and revulsion at what has happened, on the other, a need to think calmly and dispassionately about the most appropriate policy response. One does not necessarily help the other. Nato's invocation of Article 5 of the Washington Treaty is an understandable, indeed necessary, expression of support. But this is not the sort of situation envisaged by Nato's founders. The United States is not threatened in any existential sense, nor is there a readily identifiable enemy. The danger in describing this as a "war" is that discussion of the options will be based on assumptions and concepts that are inappropriate, leading to wrong policy choices.

Some of the possible responses will be justified and should command broad international support. Clear evidence that Osama bin Laden orchestrated Tuesday's atrocities will necessitate military action to eliminate the threat he and his organisation pose. In these circumstances, the Taliban regime that has provided him with a safe haven in Afghanistan would also be a legitimate target, even without proof of direct complicity. The security vacuum created by the Taliban's medieval rule is a running sore that has brought misery to the people of Afghanistan, regional instability in Asia, a huge refugee crisis and a large increase in heroin trafficking. If, by omission or commission, it now poses a major threat to human life across the globe, its existence can no longer be tolerated.

Beyond this point, great caution is needed. Dark mutterings in Washington that these attacks could not have succeeded without state sponsorship by Iraq or another "rogue state" should receive a wary response. A high burden of proof will need to be met before any action is taken against a third party. If America is seen to be using the opportunity to settle old scores or wage a wider campaign against states of which it disapproves, the result will be a greater polarisation of world opinion, increased resentment against the West and less security for Americans.

Even action against the Taliban would be fraught with difficulty. The systematic use of air power risks driving Afghans into the arms of the Taliban and would prove useless against a country that has virtually no infrastructure, military of civil. The deployment of ground troops raises the spectre of the United States being drawn into a protracted and unwinnable guerrilla war, just as the Soviets were in the 1980s. However it decides to proceed, it is essential that the United States develops the broadest coalition of international support and reigns in its instinct to lash out unilaterally.

Some options need to be ruled out categorically. The suggestion by one defence expert that US policy makers might consider fuel-laden airliners used as guided missiles to be weapons of mass destruction, to which a tactical nuclear response might be justified, lies at the most extreme end of the spectrum. But it is not out of the question. With their judgement impaired by grief and anger, and without easy conventional options to hand, it is just possible that some US officials might be tempted down this path. Crossing the nuclear threshold would be a tragic mistake for which we would all end up paying a heavy price. If necessary, Tony Blair should be ready to make that clear as Clement Atlee did to President Truman during the Korean War.

All of this underlines the danger of conceding unconditional support in advance for any course of action the US deems appropriate. Jack Straw said yesterday that no "blank cheques" had been signed and that the Bush administration would consult allies before taking any action. Tony Blair's rhetoric has suggested a rather more open-ended commitment. This may be necessary for the moment. Anything less than fulsome support at this stage risks driving the US into the sort of unilateral action that would only make matters worse.

But there may come a time when our natural impulse to rally behind our closest ally is tempered by the need to urge restraint and the Government must be self-confident enough to know when that time has come.

In the longer term, we will need to engage the US in a process of reflection on the lessons to be learned from this sickening episode. This is not simply a security issue requiring military solutions, nor is it meaningful to dismiss these suicide bombings as unfathomable acts of evil. They took place within a political context that we ourselves have helped to shape.

Something rotten has happened to relations between the West and the rest. In many parts of the developing world a deep sense of alienation has begun to manifests itself in a hatred of the industrialised countries and the US in particular. International institutions and the structures of world trade are seen to be loaded in our favour. The world's poor experience the daily immiseration of their lives, often the result of policies imposed on them by the West, while images of our good life mock them via the global media. We only appear interested in helping the oppressed when it provides cover for advancing our own interests. Add to this our double standards in the conflict between Israel and the Palestinians and you find the toxic mix of bitterness and hopelessness on which extremism feeds.

Globalisation is suffering from a crisis of legitimacy. If there is to be one positive outcome from this week's events, it must be the beginning of a new dialogue with the developing countries to address that crisis and give them a real stake in a common future. If we choose to ignore these lessons and treat the upsurge of extremism as a security threat requiring a stiff military response, we will end up creating greater division and more bloodshed. That, and not the murder of thousands of innocent civilians, would be the terrorists' real victory.

The author was special adviser to Robin Cook from 1994 to 2001