An unquiet ghost stalks the White House Situation Room as Barack Obama, increasingly Hamlet-like, ponders what to do in Afghanistan: it is the spectre of the Vietnam War, America’s enduring historical hang-up.

Comparisons between these two conflicts are easy to make, but hard to avoid: a grinding, unpredictable battle in difficult terrain, a weak and corrupted foreign government kept afloat by American guns and money; a versatile enemy, adept at ambush warfare, with sanctuary in a neighbouring country.

American public opinion on Afghanistan is shifting in a way reminiscent of the tide of feeling that brought the Vietnam War to its humiliating close. The death toll is ramping up, with 55 US servicemen killed this month — the war is suddenly being brought home to America, in bodybags.

The first senior official has resigned in protest over a war that may be unwinnable. “I fail to see the value in the continued US casualties or expenditures . . . in what is, truly, a civil war,” declared Matthew Hoh, the decorated former Marine who resigned from the US Foreign Service this week. He might have been speaking in 1969.

The most important parallels with Vietnam are neither tactical nor practical, but cultural and emotional. Americans are not backward-looking by nature, but the trauma of Vietnam is seared on the national memory like no other event in US history.

The debate is suffused with the language of the Vietnam War: “hawks”, “doves” and fear of the “quagmire”. Mr Obama is of the post-Vietnam generation, yet he, too, is haunted by it. Last month he declared: “You never step into the same river twice, and so, Afghanistan is not Vietnam. But the danger of overreach and not having clear goals and not having strong support from the American people, those are all issues that I think about all the time.”

Those words perfectly capture the anxiety that is fraying nerves in the White House, a determination to avoid the mistakes of Vietnam, but an inability to see the conflict though any other prism. After the Gulf War, the first President Bush declared triumphantly, and quite wrongly, that America had “kicked the Vietnam syndrome once and for all”.

The hawks are quick to point out that the Vietnam analogy is routinely trotted out whenever America goes to war. Over the past 25 years it has been invoked in response to US military action in Lebanon, El Salvador, Somalia, Bosnia, Haiti, Kosovo and Iraq (twice).

In terms of scale, Afghanistan is still a far cry from Vietnam, which involved more than half a million US troops and claimed 58,000 American lives. Fewer than 1,000 American troops have died in Afghanistan in eight years. In the earlier conflict, America was facing not only the Vietcong, but the highly trained North Vietnamese Army, supported by the Soviet Union and China.

When President Johnson intensified the war in Vietnam he, too, was impelled by a determination not to repeat the past, but in this case the precedent was appeasement and the lessons of Munich.

Which lessons may be drawn from the Vietnam War depend on which historians are doing the looking and which part of that long and bitter conflict they are looking at.

Two history books on the Vietnam War are at present slugging it out on Washington bedside tables. Gordon Goldstein’s Lessons in Disaster argues that Lyndon Johnson was pressed by the military into escalating an unwinnable conflict, while Lewis Sorley, in A Better War, suggests that antiwar feeling and pressure from Congress forced Richard Nixon to reject a counter-insurgency strategy that could have succeeded.

Vietnam is instructive not because it offers some neat lesson in how to win or end the war in Afghanistan, but because the American public are increasingly convinced that the latter conflict is heir to the former.

The polls are extraordinarily revealing. Some 63 per cent of Americans do not think Mr Obama has a clear plan for the war. A similar proportion oppose sending more troops. As Country Joe McDonald put it in 1965: “And it’s one, two, three . . . What are we fighting for?” The most revealing statistic of all suggests that a narrow majority of Americans now believe that Afghanistan is turning into “another Vietnam”.

The military and strategic situation in Afghanistan has not reached the critical impasse of Vietnam, but a psychological and political turning point has been reached, based on a collective, if often simplistic, memory of the Vietnam War.

The antiwar protests in the US are still small, but opposition to the Vietnam conflict also started piecemeal. For every official who resigns on principle, such as Mr Hoh, how many more, as in the Vietnam era, nurse their creeping doubts in silence? Now, as then, the fear is growing that American lives are being lost at an accelerating rate, for reasons that are increasingly obscure to many Americans, in defence of a dubious regime. News that the CIA has been bankrolling the Afghan President’s drug baron brother has compounded the sense of déjà vu.

Allied to this is concern that Mr Obama’s hopeful promises of change are being swept away by the cost of the war ($250,000 a soldier a year), in the same way that Johnson’s domestic agenda was hampered by “that bitch of a war” in Vietnam.

Mr Obama may authorise the deployment of more troops to Afghanistan; the war may yet be won by military means; fears of an Afghan quagmire may prove unfounded.

But as he ponders whether to hurl more slings and arrows into that fight, Mr Obama is facing a conflict on another front, nearer to home, harder to pin down than the Taleban, and just as dangerous to his Administration — the growing perception that history is repeating itself. Fighting the ghosts of Vietnam has become an urgent military priority. If he loses that battle, he will have lost the war.

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