Few, however, can match his invocation of Graham Greene in defence of the US's policy in Iraq.

Where Bush is the most faith-driven of leaders, so unafflicted by self-doubt, Greene is the mouthpiece par excellence of seedy ambiguity, tattered faith and human frailty.

The subject was Vietnam, the war Bush famously chose not to fight. In the intervening 30-plus years, however, he has plainly undergone something of a conversion. Vietnam was a noble undertaking, was his basic message this week to a convention of US veterans. If anything, the mistake of the US was not to stay there longer, a mistake he has no intention of repeating in Iraq.

Thus he cited Greene's 1955 novel The Quiet American and singled out one of its main characters, the US government agent Alden Pyle. Those were the days when the French still ruled Indochina but Pyle, young, driven and naive, was already on the scene, seeking surreptitiously to bring American solutions to the unhappy land of Vietnam. Many find him insufferable . . . but not Bush, as he tries in vain to bring American solutions to an equally unhappy Iraq.

In Kansas City on Wednesday, Bush used three arguments . . . two negative and one positive . . . to draw his version of the parallels between Iraq and Vietnam. A hasty American pullout, he maintained, would bring unspeakable horrors for innocent civilians in Iraq and neighbouring countries, just as American withdrawal had led to suffering and slaughter in Indochina three decades ago.

By contrast . . . and this was his second argument . . . when the US "stays the course" (to use a once-favoured but now abandoned Bushism) as it did in Germany and Japan after the second world war and in South Korea after the 1953 armistice, the results in terms of democracy and prosperity are miraculous to behold.

Thirdly, an American retreat would do huge damage to US credibility. It would be greeted by terrorists as a victory that would embolden them in their global challenge to the lone superpower, and increase the risks of new, and perhaps even deadlier, 9/11s on the soil of the US itself.

Inspect them closely, however, and each of the arguments has less substance than its predecessor. Bush was on his firmest ground when he pointed to the terrible events that followed the US's departure from Vietnam. Then, as now, he said, some insisted the real problem was the US presence, "and that if we would just withdraw, the killing would end". Instead, unknown tens or hundreds of thousands of Vietnamese were executed, imprisoned or otherwise "re-educated" in camps, and more than a million others became refugees. But these upheavals paled beside the genocide unleashed by Pol Pot and the Khmer Rouge in Cambodia, that killed some 1.7 million people, a fifth of the population.

In fact, however, these disasters were not precipitated by a rapid withdrawal of the sort opponents of the Iraq war seek. The departure of US forces from Vietnam was a long, gradual process that began in 1968 and continued for six years. As for the rise of Pol Pot, historians agree that without the war in Vietnam and the "collateral damage" visited upon Cambodia by US bombers and the machinations of the CIA, this almost certainly would never have happened.

And Bush overlooked another point. His troop "surge" may be producing some results but the political settlement that alone can produce genuine stability looks more remote than ever. In its absence, Iraq's own killing fields of innocent civilians multiply.

Two million refugees True, the victims are numbered in dozens, not thousands as in Cambodia. But that is only because millions have already fled for their lives. Some two million or more Iraqis are refugees. In the short term, the violence in Iraq might well worsen, if US troops are withdrawn. In the long run, however, consider Vietnam, today a unified, peaceful and increasingly prosperous country.

So assume the US does stay. Is there a chance of turning Iraq into a Middle Eastern equivalent of Germany, Japan or South Korea? That was the conceit of the neo-conservatives when they made the case for war. Today, that vision lies in ruins . . . for reasons Bush conveniently omitted in his Kansas City speech.

First and foremost, Germany and Japan were homogeneous countries, by 1945 crushed, exhausted and shamed, yet with huge civic, industrial and cultural traditions waiting to be revived. Iraq is an artificial amalgam of three former and very different provinces of the Ottoman Empire, held together by a tyrant. A slightly better analogy might be South Korea, which, protected by the US military shield, has transformed itself from a poor and largely rural country into a global economic powerhouse. But partition ended the Korean War. In Iraq, civil war continues to rage . . . and if the Korean timetable applies, US soldiers will still be there in 2060 trying to contain it.

Second, Bush refuses to put his money where his mouth is. His war . . . its cost of 365bn and counting already exceeds that of Vietnam . . . has seen no tax increases to pay for it. It demands no sacrifices except from those who actually fight it and their families.

At the height of the Vietnam War, half a million US troops were deployed. Even with the surge, barely a third of that number are in Iraq . . . as everyone but this president has long admitted, far fewer than are needed.

It is worth noting that in post-war Germany and Japan, there were three times as many allied occupation troops per head of the local population as in Iraq. But to provide more troops would mean a return of the same draft that brought Vietnam directly into the personal experience of millions of American families.

'Allure of retreat' The president's third point of comparison between Iraq now and Indochina then is that withdrawal . . . a surrender, in his words, to "the deceptive allure of retreat" . . . would be disastrous for US credibility.

After all, Bush noted, had not both Osama bin Laden and his deputy, Ayman alZawahiri, invoked Vietnam as a lesson.

Americans, he quoted Zawahiri as saying, "ran and left their agents" in Vietnam, and "know better than others that there is no hope in victoryf The Vietnam spectre is closing every outlet."

A withdrawal from Iraq might indeed damage US credibility . . . but if the example of Vietnam is any guide, that damage will be very slight. Probably, Moscow was emboldened in its war-by-proxy with the US, in Cuba, Angola and like places. But the dominoes did not fall in Southeast Asia.

And if Vietnam fuelled a perception of US weakness that helped persuade the Kremlin to invade Afghanistan in 1979, then it contributed to the collapse not of the US, but of the Soviet Union itself.

In fact, any loss of credibility caused by an early wind-down of the US presence in Iraq would surely be outweighed by the removal . . . or, at least, mitigation . . . of the moral stain caused by an unprovoked invasion of a country that did not represent the slightest threat to the US and had nothing to do with 9/11, and by the subsequent disgrace of Abu Ghraib.

Vietnam was a conflict that gradually escalated from small beginnings, as Washington's military strategists fell into the trap of believing just one more shove would do the trick. Iraq was a war of choice, and as such will probably go down in US history as an even greater blunder than Vietnam.

Flawed premises Of course, these two wars, though more than a generation apart, do have similarities. Each of them was based on a flawed premise. Vietnam was supposed to be part of the free world's global battle against communism. In fact, the north's ultimately successful campaign to unify the country was the final act of a decades-long, anti-colonial struggle.

Iraq was, and continues to be, presented as a central front in the "war against terror", made even more threatening by those imaginary WMD Saddam Hussein was reputedly aiming at the US's jugular.

As even Bush now grudgingly admits, Saddam had nothing to do with 9/11. In fact, his secular regime was anathema to Bin Laden. If Iraq is now a terrorist breeding ground, that is entirely of this president's making.

And underlying US instincts have not changed. In The Quiet American, Alden Pyle reckons he has all the answers . . . above all that the American way alone can bring good things like "national democracy" to the unhappy land of Vietnam.

To which the older, wiser and deeply sceptical British journalist Thomas Fowler replies that the real aspirations of the Vietnamese are less exalted. "They want enough rice. They don't want to be shot at.

They don't want our white skins telling them what they want."

But it is hard not to see Bush as a modern Pyle, with the same disastrous blend of idealism, ignorance and wellmeaning arrogance.

"We are the old colonial peoples, " Fowler, the authentic voice of Donald Rumsfeld's despised 'Old Europe', tells Pyle. "But we've learnt a bit of reality. We have learned not to play with matches." And was ever a larger match tossed into a greater sea of petrol than that of George Bush's invasion of Iraq?