In the pre-Iraq years, attitudes to war on both sides of the Atlantic were commonly framed by one of two radically opposed mythic experiences. A supportable war was the sort embodied in Britain's defiance of Hitler in 1940, whose lesson was that the right people would win if they stood firm against evil. An unsupportable war was encapsulated in America's rout in Saigon in 1975, whose lesson was that conflicts were more complicated in practice.
When George Bush decided to invade Iraq, he offered Americans a rerun of a 1940-style war, with himself in the role of Winston Churchill and Tony Blair as his transatlantic cheerleader. Today, as Bush's surge strategy - the deployment six months ago of a further five combat brigades - struggles to produce the promised dividends, Iraq has flipped into the alternative frame. It is widely assumed that the conflict is heading inexorably to a 1975-style nemesis.
But is this really true? Do these potent precedents illuminate the only possible alternatives? At the very least, such questions must be examined. The US is better at doing this than Britain, since our role is in any case reduced and detailed public discussion of Iraq options rare. One problem is that much of the discussion in 2007 is a continuation of the argument about what ought to have been done in 2003. Defenders of the surge may no longer talk about creating a democratic Iraq that will transform the region; but they still talk about staying the course and doing what it takes to achieve some vaguer benign goal. Opponents go to the other end of the spectrum, however, and say that the best thing is for the US and its allies to get out of Iraq as quickly as possible, frequently without regard to the practicalities or wider consequences.
In their different ways, both surgers and withdrawers need to have their assumptions more critically examined in the light of today's realities. The White House's recent claim that the surge is developing satisfactorily is mocked by the very modest security advances and the almost total absence of political advances. As General Wesley Clark said last week, the US would have had to triple the number of troops on the ground to have the effect that the administration seeks as its goal. But it does not follow that outright withdrawal is therefore the only alternative worth considering. Or that it is politically popular: only 36% of Americans want all the troops removed from Iraq, while support for the war has actually increased in the latest US polls.
Withdrawal may sound straightforward, but it isn't. Although the popular image of the end of the Vietnam war is of a sudden disorderly helicopter evacuation from the roof of the US embassy in Saigon, the reality is that the process took more than four years. It took the Soviet Union nine months to pull 120,000 troops out of Afghanistan in the early 1980s - and those two countries shared a land border. One recent estimate for extricating the more than 200,000 US troops, contractors and foreign workers now in Iraq is that the whole process would take at least a year.
And that is just the human dimension. The US also has an enormous amount of stuff in Iraq, including around 1,900 heavy tanks, 43,000 other military vehicles and more than 700 aircraft, mainly helicopters. Then there are the houses, messes, fitness centres, hospitals, swimming pools, office complexes, power plants, kit and supplies that the Americans brought with them - one current military inventory lists 2.7m candy bars and 1.6m cans of soft drink. It is all scattered across 15 large and 60 smaller bases, 28 supply depots and other sites. At four bases, there are toxic stockpiles of hazardous materials. A full withdrawal of people and kit would take an estimated 20 months.
Getting it all out would be no picnic, especially if airfield runways are under mortar attack and roads to the south are mined. Abandoning and disabling it may well be the only practicable course. The prospect of the US having to fight its way out of Iraq is a very real one. And even if the retreating Americans could withdraw in good order, the local death toll would be likely to rise as competing factions battled to fill the vacuum. Withdrawal may be the right long-term policy option for the US - but it will be a very bloody and traumatic business, in whatever way it is accomplished.
That is why there is in Washington increasing examination of other strategies. It is some years since the words "third way" were heard in the land, but that is the title of an investigation into partial withdrawal options now being carried out by the US House of Representatives armed services committee. American newspapers and magazines have also carried detailed and thoughtful articles on these subjects - including last week's Time magazine.
Partial withdrawal sounds beguiling, centrist and moderate. Yet you only have to look at what these third way strategies might mean on the ground to see that they are not as attractive as they sound. Stephen Biddle, from the Council on Foreign Relations, identified four partial withdrawal options - all of which would reduce US numbers by half - in his evidence to the congressional committee: withdrawal towards training and support of the Iraqis, withdrawal to guarding Iraq's borders, withdrawal into a focused mission on counter-terrorism, and withdrawal into Kurdistan.
The Kurdistan option is eloquently expounded by Peter Galbraith in the current New York Review of Books, but Biddle himself is not optimistic about any of the four choices. He thinks they are all self-defeating and unsustainable. "Partial withdrawal would not end American casualties," he says. "But it would make it even less likely that the lives we do lose would be lost for any purpose, or in exchange for any improvement in the future of Iraq."
If that is so, then the choice may after all be between the extreme alternatives of sustained military commitment and total withdrawal. But only because one of these is the elusive least worst option, not because it is a good or wise policy. There is an overwhelming case for the Pentagon and the White House to set out the full and true contingency options. It would be reassuring if Gordon Brown were to take such a message to Bush at Camp David this weekend. And even more surprising if Bush was to listen. Bush started this war on the basis of inadequate planning. It looks as if he will end it in the same way too.