By James Harding

With the launch of a military attack on Baghdad, George W. Bush has transformed America's mission in the world with a war of his own choosing. The Iraq conflict of 2003 is the first fought under the doctrine of preventive war. It is waged not only with the aim of changing a regime but also with the ambition of converting a region to democratic values.

It is a fight that inflicted huge collateral damage before the first salvoes were fired: the United Nations and Nato appear to be sorely weakened and allied governments from London to Ankara to Islamabad destabilised. The US - a nation that sacrificed blood and wealth in the wars of the 20th century and invested colossal energy in constructing the institutions of the postwar world order - now finds itself feared in some parts of the globe as a headstrong, even bellicose, imperial power.

"It is," says Henry Kissinger, the former secretary of state, "a defining juncture in American foreign policy . . . a historic turning-point." In this era of terrorism and weapons proliferation, the architect of American realpolitik endorses the use of force in Iraq. But, acknowledging the rupture to inter- national institutions and strategic alliances, Mr. Kissinger adds: "It will require the re-examination of the basic assumptions of the last 50 years."

The diplomatic damage of the past six months, to relations with friend and foe, need not be permanent. Mr Bush's decision to take pre-emptive action may not prove the template for all US wars of the 21st century. But the second Gulf war is plainly much more than a re-run of the first.

The transformation stems, of course, from the terrorist attacks of September 11 2001. When hijacked aircraft felled the twin towers, America felt violated by forces beyond its control. With the invasion of Iraq, Mr Bush aims to recapture the initiative.

It is true the cast is uncannily similar: Mr Bush, the 43rd president, is advised by many of the people who steered his father George H.W. Bush, the 41st president, to victory in 1991. Dick Cheney, secretary of defence 12 years ago, is a vice-president central to the staffing of the Bush presidency and advancing the argument for regime change in Iraq. Colin Powell, secretary of state, was chairman of the joint chiefs of staff in 1991. And throughout the upper echelons of the Pentagon, the National Security Council and the White House there are officials who steered the US to war once before with Mr Hussein: Paul Wolfowitz at the Pentagon and Stephen Hadley at the National Security Council are just two.

The images of US and British troops in desert fatigues, in the same sandy theatre and against the same rifle- toting adversary - all captured in breathless newscasts on CNN - add to the sense of déjà vu.

Even the script is laden with familiar phrases. George W. Bush addressed the American people on television sitting in the same chair his father occupied when he announced the start of air strikes in 1991. The president declared a coalition of 35 countries was ranged against Mr Hussein, stressed that "we come to Iraq with respect for its citizens" and pledged to "pass through this time of peril" and "prevail" - much as his father had announced a coalition of 28 countries, emphasised that "we have no argument with the people of Iraq" and promised that the US would "prevail".

Yet the echoes are misleading. The renewed assault on Baghdad is not simply an attempt to tidy up unfinished business. Indeed, the younger Mr Bush's war in Iraq is in many ways the antithesis of the one his father fought in 1991. Twelve years ago the US responded to an act of aggression - the invasion of Kuwait. Today, it is enforcing a new military philosophy. "Instead of drifting along toward tragedy, we will set a course toward safety," Mr Bush declared. "Before the day of horror can come, before it is too late to act, this danger will be removed."

The White House maintains that pre-emption is just one tool among many to be used to maintain national security. Mr Bush has had no choice, the argument runs, but to resort to preventive strikes in a frightening new world where terrorists could obtain weapons of mass destruction courtesy of tyrants such as Mr Hussein.

"The first thing to say about pre-emption is that it is not a new concept; that you don't have to wait to be attacked to try and diminish a threat against you is not a new concept," Condoleezza Rice, the president's national security adviser, said when the concept was written into the US national security strategy last year.

To many others, however, the military precept seems to rewrite 50 years of US defence policy. The Congressional Research Service, a non-partisan group, last year considered all military action taken since the founding of the Republic and concluded: "The historical record indicates that the US has never, to date, engaged in a "pre- emptive" military attack against another nation." (A possible singular exception, it allowed, was the US's one imperialist adventure, the Spanish-American war of 1898.)

Certainly, 20th-century American leaders were vocal in rejecting the arguments for pre-emption. "In my opinion, there is no such thing as a preventive war," Dwight D. Eisenhower said when war flared on the Korean peninsula in 1950. "The fact [is] that war begets conditions that beget further war . . . When people speak to you about a preventive war, you tell them to go and fight it."

Just over a decade later, Robert Kennedy rejected the idea of pre- emption in the febrile days of the Cuban missile crisis: "For 175 years," he said, "we have not been that kind of country."

Today, the mandarins of Washington's foreign policy establishment understand the rationale behind pre-emption in an America awakened to its vulnerabilities by the terrorist attacks of September 11. Sandy Berger, the national security adviser under Bill Clinton, says: "9/11 is one of the reasons why Americans see this differently from Europe. After 9/11, uncertainty is a reason to act for Americans . . . Americans saw on 9/11 that what we don't know can hurt us."

Even so, there is deep disquiet about the example America is setting for the world. Zbigniew Brzezinski, Jimmy Carter's national security adviser, worries about the spread of the Bush doctrine: "The obvious risks are that pre-emption which entails unilateral judgments could be infectious. Should India consider that as a model for dealing with Pakistan?"

Abroad, Washington's uninhibited and unapologetic exercise of power has generated unprecedented opposition. Millions have taken to the streets. Supposed allies have turned away. Vladimir Putin, the Russian president with whom Mr Bush is said to have cultivated a personal rapport, said on Thursday: "Military action can in no way be justified. Military action is a big political error."

Kofi Annan, the UN secretary general, who just a couple of months ago was Mr Bush's guest at the White House for a private dinner, insisted Iraq could have been disarmed peacefully, if diplomacy had been allowed to continue:

"Or, if not, the world could have taken action to solve this problem by a collective decision, endowing it with greater legitimacy and therefore commanding wider support than is now the case."

In 1991, the elder Mr Bush could announce on the first night of the war an international army comprising troops from 28 countries, among them Muslim soldiers from Egypt and Saudi Arabia. One person who sat in on the tense discussions about military action in the Oval Office 12 years ago recalls: "We had a debate about what would be a casus belli. And we decided that a UN resolution would provide the cover."

The current president has scrambled to put together a larger coalition of willing countries but the combat forces essentially come from three: the US, the UK and Australia. The White House paints Mr Bush as a champion of the world body, a politician seeking to give meaning to the resolutions long defied by Mr Hussein.

In the six months since Mr Bush took his case to the UN the reasons for disarming Iraq have shifted. The White House has raised fears of nuclear weapons, presented evidence of chemical and biological capabilities, listed the litany of Mr Hussein's crimes against his own people and, with a remarkable degree of success in US public opinion, forged the notion of a link between Iraq and Osama bin Laden's al-Qaeda network.

Enforcing the demands of the UN set out over more than a dozen resolutions has been the Bush administration's most unwavering and legally watertight justification for war. But the failure of the diplomatic effort to secure UN support for a second resolution specifically authorising war against Iraq - not to mention the divisions created with and between European allies and the arguments sown in Nato and the European Union along the way - alarms some foreign policy grandees who spent the bulk of their careers creating and cultivating institutional alliances.

"The run-up to the war has opened a number of fissures in the international system with the UN, with Nato, with the EU and with a number of our allies," says one former senior US administration official. "The war is likely to add turmoil, if not increase the number of fissures."

Mr Bush's assertion on Monday night that "the Security Council has failed in its responsibilities" is particularly disturbing, the official argues. It presents the US as "some independent judge off to the side pointing at some different entity . . . It [the UN] was the entity which provided the legitimacy in the Gulf war."

The rupture, arguably, did not begin with this war. When Mr Bush took on the Taliban and al-Qaeda in Afghanistan in 2001, he preferred to convene his own coalition rather than operate through Nato. But the campaign against the Taliban was widely applauded, and joined, by the US's allies. With the creation of a second ad hoc coalition in two years, a superpower that has operated since the second world war through the UN, through Nato and through the Bretton Woods institutions now appears willing to treat such organisations as optional, even expendable.

"For the last 50 years, we had a treaty based internationalism, we led coalitions grounded in institutions," says John Hamre, former deputy defence secretary, now president of the Centre for Strategic and International Studies, Washington. "Now we are based on a consensus-based, coalitions of the willing, internationalism."

"Consensus-based internationalism is arguably more flexible," says Mr Hamre. "But the question is whether it is sustainable over time."

Forming coalitions of the willing has freed Mr Bush to declare a far more sweeping purpose in the Gulf than his father allowed himself. The 41st president wanted to show that national boundaries would be respected, that Iraqis would be forced to leave Kuwaiti territory and that "the rule of law, not the law of the jungle, governs the conduct of nations". His son describes a broader campaign to bring liberty to the Arab world.

In a speech last month to the American Enterprise Institute, a conservative think-tank, Mr Bush articulated his aims thus: "A liberated Iraq can show the power of freedom to transform that vital region, by bringing hope and progress into the lives of millions." A free Iraq, he argued, would act as a catalyst for the creation of a Palestinian state and inspire nations in the Arab world to embrace democracy.

A sceptical Mr Berger calls it the "dominoes rising" theory: "If you get the Iraqi domino set up straight, it will help set up the other dominoes in the region."

This transformationalist approach is widely viewed as a favourite project of the neo-conservative wing of the Republican party. But not all hawks are neo-conservatives. And not all neo-conservatives want to remake the Middle East. Mr Wolfowitz is seen as the intellectual champion of those conservatives who want to use American power as a moral force to spread freedom; Donald Rumsfeld, his boss at the Pentagon, is regarded as a more traditional nationalist, willing to send in the US military to erase a threat and then, promptly, return home.

What George W. Bush does share with his father is the public suspicions of his motives, namely that he is waging war for oil or domestic political advantage. Some giant contracts may fall into the laps of US infrastructure and energy groups with connections to the Bush administration, such as Mr. Cheney's old employer Halliburton. But the US has been at pains to ensure that revenues from Iraqi oilfields will flow to Iraqis.

Nor is the war driven by hopes of electoral gain. If the sands of southern Iraq prove a military quagmire, domestic disquiet could cost Mr Bush his presidency, much as Vietnam ended the career of Lyndon Johnson. Even a quick victory could leave Mr Bush in much the same position as his father, who learnt, like Winston Churchill, that success on a battlefield far from home does not guarantee the affection of the voters.

"A successful war merely means that the next election will be fought on domestic issues," says a senior Republican figure in Washington, with an anxious eye to the uncertain US economy.

Success, though, is what the Bush administration predicts will redeem the US in the eyes of the doubters and critics. "Things may change rather markedly once success comes, this threat is removed and the Iraqi people face a better future," Mr Powell said earlier this year. "We will be successful - and from that success new opportunities will arise."

But the extraordinary paradox of a precedent-setting war is that even the prospect of success is met with mixed feelings. Everyone hopes for a short war with minimal casualties but that is not all that hangs on the outcome.

"If the war ends up being a cream puff - in the sense that everything goes smoothly in terms of the military undertaking and the aftermath - it will probably create temptations to act elsewhere," says Mr Brzezinski, noting the ambitions of some conservatives to see a democratic transformation of Syria and Saudi Arabia, as well as the Bush administration's unresolved disagreements with the other "axis of evil" nations, Iran and North Korea.

The harder the conflict is, the less attractive will be the argument that this is a new paradigm for American leadership. But, says Mr Brzezinski, "if the war creates an intoxicating hubris, we could have a spiral which could be very damaging."