According to the neo-conservative Washington Post columnist Charles Krauthammer, from the end of the Cold War until the terrorist attack of September 11, 2001, the United States took a 10-year "holiday from history".

On the face of it, this seems a strange way to characterise American behaviour during the decade. US military forces were more active during these years than at any time since the Vietnam War. The American economy enjoyed a sustained six-year boom, easily outperforming both the Japanese and European economies. On Washington's initiative, NATO expanded eastwards towards the Russian border. The North American Free Trade Agreement was negotiated and the World Trade Organisation established.

Given all this, what does it mean to say that the US had taken a "holiday from history"? What Krauthammer meant was that during these years, the US, having become the sole remaining superpower and an authentic global hegemony, had failed to define and activate a grand purpose or mission commensurate with that status and the opportunity it presented.

No such thing was evident during the last decade of the 20th century. True, George Bush the elder did have a shot at it. But as he himself confessed, he wasn't very good at what he called "the vision thing", and his concept of a "new world order" was stillborn.

His successor, William Jefferson Clinton, was a brilliant intuitive politician, an improviser with little taste for doctrines or vision. Clinton was a pragmatist, a compromiser, a deal-maker, a triangulator of differences. A connoisseur of opinion polls and focus groups, he knew that in the postwar period Americans consistently gave foreign policy a low priority. Clinton acted accordingly, taking a limited interest in foreign policy.

While America has enough strength to defeat all other adversaries and rivals, it remains to be seen whether she can take on and conquer Utopia.
Those who looked for a grand unifying purpose for American foreign policy were left frustrated and unhappy by Clinton's eight years in office.

In January 2001, George Bush succeeded Clinton. When a new president comes to office, especially when he is of a different party, there is a settling-in period that takes months. This process had barely finished when the terrorist attack occurred on September 11. How the Bush Administration's foreign policy would have developed in the absence of that attack, we shall never know, and what evidence we have is ambiguous.

In any case, with the attack of September 11, America's alleged "holiday from history" came to an abrupt end. In an instant the terrorists had given the country the clear purpose, the central organising principle that it had previously lacked and that some had been strenuously demanding.

One of the effects of September 11 was that it shifted the balance in favour of those in Washington's foreign policy establishment who saw things in sweeping terms - away from prudence and moderation towards conceptual boldness and an ambitious, assertive use of American power. Within a year the "war on terror" had metastasised into something much grander and more radical; something that would give full expression to one of the strongest strands in the history of the American people: the profound belief, that is, that they and their country are destined to reshape the world.

There were many in and around the Bush Administration who shared this sense of America's destiny. They now saw in September 11 not merely a disaster to be avenged but an opportunity to reawaken and redirect America to its true historic mission.

In the aftermath of September 11 those who thought in these terms came into their own. The result became fully evident with the publication a year later of a 31-page statement by the President titled The National Security Strategy of the United States of America. In my judgement, this document is without a doubt the most important statement about American foreign policy, not just since the terrorist attack, and not just since the end of the Cold War, but since the enunciation of the Truman Doctrine in 1947. For in it is spelt out how the US intends to use its hegemonic power.

What can we say about this strategic doctrine? Well, the first thing to be emphasised is its breathtaking scope, its huge ambition to do no less than to effect a transformation of the political universe - according to some of its language, to stamp out evil and war between states, to create a benign world.

Students of international politics who belong to the realist school - as I do - tend to see such goals as utopian, beyond even the reach of a country with the enormous power of the US. While America has enough strength to defeat all other adversaries and rivals, it remains to be seen whether she can take on and conquer Utopia.

Second, in emphasising and insisting upon the dominant role of the US and the assertive use of American power, the doctrine makes very questionable assumptions about what the other states will accept and live with. They are asked to take its good intentions and respect for their interests on trust. States have never been prepared to do this in the past with other would-be hegemonies.

Will the US be the exception? Does the fact that it is a democratic and liberal state make a decisive difference? Will other states accept the concept of a benign hegemony or regard it as a contradiction in terms? Indeed, do they have a choice?

The thrust and tone of the doctrine reject the advice given by most pundits on the best way to play a hegemonic role in order to prolong its duration - which is to be restrained and prudent in the use of its power, to disguise it, to strive to act as far as possible by persuasion and consensus to co-opt others. Coral Bell of the Australian National University sums up that advice by saying, "the unipolar world should be run as if it were a concert of powers" - as if, that is, it was really multipolar.

In the 1940s, when the US was already the dominant power within the Western Alliance, it acted on this advice. It went out of its way to act multilaterally, to create a network of rule-making institutions - the UN system, the IMF, the World Bank, the GATT - that allowed it to act co-operatively with others, as the first among equals. There is little of this to be found in the current doctrine, no talk of creating institutions to run the new order. The emphasis is overwhelmingly on the mission.

The Bush doctrine should be taken very seriously and any inclination to treat it as mere rhetoric would be a serious error. It has already been put into effect in Iraq. The four standout features of the doctrine were evident there: the use of American military force as the main instrument; pre-emptive action; a clear indication that the US was prepared to act without a great power consensus, and unilaterally if necessary; and the avowed intention to replace a tyrannical regime with a liberal representative government.

That is why the Iraq commitment has an importance that goes way beyond the fate of Iraq itself. If, in the end, it turns out successfully, it is likely that the mishaps that have occurred since the end of the heavy fighting will be seen as part of a learning experience, a breaking-in period for a new, revolutionary, strategic doctrine.

If, on the other hand, it fails at the first hurdle - if, that is, the US finds that bringing about security, stability, a decent political order, and an improvement in the living standards of the Iraqi people, is beyond its capacity; if the whole thing becomes a "quagmire", or, indeed, if it has to internationalise the whole project by giving the UN a pre-eminent role - then not only will there have to be a reconsideration of the whole global strategy, but the limits of America's capacity will have been made evident, and the inclination to resist it greatly strengthened.

All this is fully understood by the advocates and supporters of the policy. The editors of the influential neo-conservative magazine The Weekly Standard, for example, insist that: "The future course of American foreign policy, American world leadership, and American security is at stake. Failure in Iraq would be a devastating blow to everything the United States hopes to accomplish, and must accomplish, in the decades ahead."

As for the sceptics and critics, some of them will conclude that having committed itself so far, the US now has no option but to go on and see it through - an argument that prevailed for a long time during the Vietnam War.

Others will argue that even at this late stage, it is preferable to cut one's losses than to proceed further with a deeply flawed policy, citing the old saw, "If you're in a hole, stop digging".