Making it clear he will resist congressional pressure next month for an early withdrawal, he signalled that US troops, whom he hailed as the "greatest force for human liberation the world has ever known", will be in Iraq as long as he is president. He also said the consequences of leaving "without getting the job done would be devastating", and "the enemy would follow us home".

Mr Bush's speech came on the day that the US suffered one of its highest daily death tolls since the 2003 invasion, with 14 troops killed when a Black Hawk helicopter crashed.

In a speech to army veterans in Kansas City, Mr Bush invoked one of the US's biggest military disasters in support of keeping troops in Iraq: "One unmistakable legacy of Vietnam is that the price of America's withdrawal was paid by millions of innocent citizens whose agonies would add to our vocabulary new terms like 'boat people', 're-education camps' and 'killing fields'."

The speech was aimed primarily at what White House officials privately describe as the "Defeatocrats", the Democratic congressmen trying to push Mr Bush into an early withdrawal. The issue is set to come to a head next month when the US commander in Iraq, General David Petraeus, gives a progress report to Congress.

Gen Petraeus is expected to say that the surge has produced military successes but that there has only been limited progress on the political front.

In relation to the latter, Mr Bush was forced yesterday to backtrack after 24 hours earlier expressing frustration with the Iraqi prime minister, Nour al-Maliki. Alarmed by the harsh reaction of Mr Maliki, Mr Bush hurriedly rewrote his speech to praise him: "Prime Minister Maliki's a good guy, a good man with a difficult job and I support him."

The speech overall reflected the White House belief that it is shifting American public opinion behind the surge - the injection of 30,000 extra US troops into Iraq that has brought the total US force in the country to its highest level, 165,000.

The Bush administration wants to keep the surge going until at least next April, at which point the overstretched military will be forced to begin reducing troop numbers anyway.

Although Gen Petraeus has not yet completed his report, a Pentagon source said the US presence could be down to 110,000 by the end of next year. The army, as of yesterday, had no plans to replace five brigades, each consisting of 3,400 to 4,000 soldiers, when their 15-month tours expire next summer.

Freedom's Watch, a conservative group, yesterday launched a $15m (£7.5m) advertising campaign in 20 states saying: "It's no time to quit. It's no time for politics."

Mr Bush's former White House spokesman, Ari Fleischer, who works for the group, said: "We want to get the message to both Democrats and Republicans: don't cut and run, fully fund the troops, and victory is the only objective."

The White House has been emboldened by a Gallup poll published yesterday showing approval ratings for the Democratic-led Congress had dropped to 18%, the lowest since the survey of the public views of the legislature began in 1974, and an earlier Gallup poll showing support for the surge had jumped in a month from 22% to 31%.

Two of the most influential senators on military affairs, the Democratic chairman of the armed services committee, Carl Levin, an advocate of an early withdrawal, and John Warner, a veteran Republican who recently broke ranks with Mr Bush over the war, issued a statement this week lauding the surge's "tangible results".

Mr Bush, until yesterday, had strenuously avoided making explicit references to Vietnam. It is a gamble, risking reminding Americans that Vietnam was a military quagmire and reminding them of the shambolic retreat from the embassy rooftop in Saigon on the day that a Black Hawk crashed in Iraq killing 14 US soldiers.

But Mr Bush tried to turn the argument around as he made a series of contentious political parallels. He argued that US involvement in the far east had turned it from a continent in 1939 with only two democracies - Australia and New Zealand - into one where democracy was the norm: he mentioned Japan, South Korea and Vietnam.

"In Cambodia, the Khmer Rouge began a murderous rule, in which hundreds of thousands of Cambodians died by starvation and torture and execution," Mr Bush said.

Some historians argue that it was the US covert bombing of Cambodia that produced the Khmer Rouge rather than US withdrawal from Vietnam.

Mr Bush added: "In Vietnam, former allies of the United States and government workers and intellectuals and businessmen were sent off to prison camps, where tens of thousand perished. Hundreds of thousands more fled the country on rickety boats, many of them going to their graves in the South China Sea."

He said that there had been lots of critics of US involvement in Vietnam at the time. But he quoted from Graham Greene's novel The Quiet American, the words "I never knew a man who had better motives for all the trouble he caused", implying that, with the benefit of hindsight, they were wrong, just as critics of the Iraq war will later seen to be misguided.

He will expand on that in a speech next week in which he will say he has not abandoned his ambitious idea that Iraq could be in the vanguard of bringing democracy to the Middle East.

Since the British government hinted recently that it planned an early pullout from Iraq, it has come under increasing pressure from the White House. US general Jack Keane yesterday became the latest American to criticise the proposed British move.

He told the BBC that the situation in Basra was deteriorating. "From a military perspective I know what the [US] commanders are trying to avoid is having to send reinforcements to the south from forces that are needed in the central part of Iraq. That situation could arise if the situation gets worse in Basra if and when British troops leave," he said.

Literary allusions

Bush's "better motives" quote comes from Graham Greene's The Quiet American, a searing attack on US foreign policy set in Saigon in 1952. It is the story of jaded British reporter Thomas Fowler and his relationship with younger US spy Alden Pyle, told against the backdrop of the French battle with the Viet Minh - precursor to the Vietnam war. "Innocence is a kind of insanity," says Fowler of Pyle as he blunders into the conflict, sponsoring a corrupt militia leader based on real nationalist Trinh Minh Thé. Fowler's jealousy over Pyle's interest in his Vietnamese mistress and distaste for his methods culminate in a bomb attack in Saigon. A film version was scheduled for release in 2001, starring Michael Caine. It was test screened on September 10 but postponed for a year after the 9/11 attacks.