The one word in every American's mind: Vietnam

The domino theory turned out to be a misconception. Are the assumptions behind this war also misconceived?

So great was the shock of American defeat in the Vietnam War 35 years ago that it remains the unseen but deeply felt influence on everything that is happening now in Iraq. Everything. When George Bush insists that the United States will obtain its objective, however long it takes, that is because he remembers the nightmare endured by his democratic predecessor in the White House, Lyndon B Johnson.

On 31 March 1968, after 50,000 American troops had been killed in an unsuccessful defence of South Vietnam against its impoverished but well-led northern neighbour, Johnson addressed the nation from the Oval Office. He announced that he would halt much of the bombing of North Vietnam and agree to negotiations with Hanoi. He did not wish to devote one hour of the coming days to partisan causes. "Accordingly, I do not seek, and I will not accept, the nomination of my party for another term as your President". In effect, the author of the ambitious Great Society programme of social reform had been run out of office by a small communist state. George W Bush isn't going to be sent packing by Saddam Hussein.

Equally, whether he realises it or not, the Iraqi dictator's defence strategy has its origins in Vietnam. He may admire Stalin, but Saddam appears to have adopted Mao's rules which so influenced the North Vietnamese generals: enemy advances, we retreat; enemy halts, we harass.

But the comparison with North Vietnam should not be taken too far. For the weaknesses of Saddam's defence of Iraq have been more reminiscent of the hopeless leadership of South Vietnam. When US advisers proposed a more rational disposition of South Vietnam forces, the President, Ngo Dinh Diem, objected that it would take away troops who defended him against palace coups and give new authority to generals he didn't trust. Mr Diem told his favourite officers not to incur casualties fighting the local communist forces. Their job was to protect him against rebellion. In his fear of coups, President Saddam is much more like the corrupt Diem than the North Vietnamese leader, Ho Chi Minh.

The principles upon which America's Vietnam policy was founded were as clear and obvious to policy makers of the time as they are now, albeit different. Instead of today's need to rid the world of weapons of mass destruction in the hands of rogue states, then the justification for using US troops to prevent a communist take-over of South Vietnam was the domino theory. Nikita Khrushchev, the Russian leader, had said that the Soviet Union would support wars of liberation wherever they might occur. As early as 1953 Richard Nixon described South Vietnam as the first in a line of dominos. If it fell to communism, the rest of Asia would be lost.Dean Rusk, the Secretary of State for John F Kennedy and Johnson, said that the US was a Pacific power as well as an Atlantic one. "To us the defence of South Vietnam has the same significance as the defence of Berlin ... it is all part of the same struggle – to prevent an extension of communist influence."

Yet the domino theory, so fervently believed, was a misconception. When South Vietnam fell to the communist North Vietnam in 1975 two years after the last US combat troops had left, nothing happened. Will the assumptions underlying US policy now turn out to be equally misconceived?

Nixon won the November 1968 election. In the period between his victory and inauguration, Nixon called on Johnson and Rusk in the White House. He told them that he admired the way "all of you have stood up through up through great fire. This is a hard time. Where was the war lost?" Rusk answered straight away: "In the editorial rooms of this country".

Which is to say that the tens of thousands of US deaths together with television reports of atrocities perpetrated by American troops, inescapable as some may have been in war, were too much to bear. In August 1965, for instance, cameras filmed a squad of US Marines burning down a village built of brick and straw. The reporter commented that this was a microcosm of the war. US firepower could win militarily, "but to a peasant whose house meant a lifetime of backbreaking labour, it will take more than presidential promises to convince him that we are on his side." It was the cumulative power of such images which led to the White House being surrounded daily with protesters chanting "Hey, hey! LBJ! How many kids did you kill today?"

If Mr Bush and Mr Blair are to have similar moments in Iraq, they won't come yet. They are more likely to arise after Saddam's regime has been removed and the coalition settles down to provide an interim government. They would come if resentment at foreign occupation boils over into terrorist activity, suicide attacks and the like and evokes a brutal response. Then everything would depend on what the electorates in the two countries felt.

For, in the public mind, a line does exist. On the one side, civilian casualties and military losses are held to be unavoidable from time to time. On the other side, a level of horror and calamity can be reached which is intolerable. The lesson of Vietnam is that if the coalition crosses this line, then it will have failed and the political reckoning will be severe.