First it looked as though the central issue of the US presidential election would be Iraq. Then it seemed the economy was the story. Now, six weeks before the election, another debate dominates the presidential race. It involves Vietnam but is not the Vietnam debate most political strategists predicted.

Recall the expectations of leading Democrats in the run-up to this election year. Everyone remembered the success of John McCain in 2000, including his unexpected popularity among Democrats. The McCain example made another hero - John Kerry, with his silver and bronze stars and three purple hearts - seem especially attractive. Democratic leaders were, however, also convinced that anti-war activists could be winners as well. After all, most senior Democrats - and the majority of journalists who cover them - long believed that the Vietnam war was a mistake. Here again, Mr Kerry, a prophet of the anti-Vietnam war movement, fitted the bill. The tape of a highly believable young Mr Kerry's interview with Dick Cavett, the television host, and his 1971 protest testimony about "winter soldiers" -were both regarded as pluses. Three decades ago Mr Kerry told Congress that the idealistic side of the battle was pointless, that in Vietnam "we found most people didn't even know the difference between communism and democracy". Mr Kerry also spoke about how other veterans had told him of mass war crimes by US soldiers.

In the context of Iraq - and especially the torture revelations of Abu Ghraib prison - quotations such as these seemed to give Mr Kerry enormous authority. He would expose the folly of George W. Bush's war as could no other. A McCain, but better. This is not how it is playing out. Mr Kerry's protest role is resonating but, instead of helping, it is damaging him, and even undermining his reputation as a war hero. Fellow veterans who have long resented his anti-war protests are now trying to prove that Mr Kerry misrepresented his military service. And they are succeeding to some degree. Instead of Mr Kerry's campaign being about his Swift Boat service, it is about angry Swift Boat veterans and their attacks on him.

Similar surprises turn up in Mr Bush's case. The president was not a protester and he was not a war hero: he served at home in the National Guard, and erratically at that. Republicans at one point considered this record a weakness. Yet it seems to be less damaging than they supposed. Dan Rather, CBS television's anchorman, made a documentary charging that Mr Bush's shortcomings while in the service were covered up. But instead of the intended result - a scandal about Mr Bush and favouritism - Mr Rather has brought trouble on himself, including charges of failing to spot false documents. The scandal has become Mr Rather's, not Mr Bush's.

Observers, especially those in Mr Rather's generation, blame the "rightwing media" for these events. Others have argued that the Swift Boat phenomenon is the result of shifts in campaign finance legislation. It is certainly true that McCain-Feingold, the new campaign-finance law, redirected cash away from parties and towards non-profit groups, which can sling mud with less consequence.

Still, the main reason the Vietnam debate has taken the turn it has is generational. In the years after Vietnam, Americans could, if they chose, comfortably tell themselves that war must be avoided at all costs. The war they were in, the cold war, seemed distant and cynical. That began changing with the fall of the Berlin Wall in 1989, the first Gulf war and, especially, the conflict in Yugoslavia. The end of the cold war proved that the fight for freedom - the phrase had seemed so tired - was perhaps worthy after all. The first Gulf war demonstrated that a war that was merely about borders and sovereignty could be an incomplete war - even a dangerous failure.

The Balkan wars and subsequent international effort, including pressure to bring down Slobodan Milosevic, showed that a moral campaign to re-establish democracy could be worthwhile. These wars formed the young adulthood of Americans born after Saigon fell.

More recently - especially after the September 11 attacks - the US accelerated its re-evaluation. Afghanistan and Iraq seemed worthwhile for moral reasons. Perhaps, people began to think, Vietnam had been worthwhile as well. The Bush administration has formulated its democracy-building programmes for Afghanistan or Iraq poorly. But the idea that the US should bring democracy to troubled places has taken root in the US - and not merely among the so-called neo-conservatives.

Evidence of the shift also came in the outpouring of grief at the death of Ronald Reagan. To lionise Reagan as the mourners did is, after all, to lionise a man who insisted, very like the Swift Boat veterans, that the Vietnam war was a "noble cause" and the US withdrawal a tragic betrayal. And with American soldiers under fire in Iraq today, Americans see Mr Kerry's harsh criticism of soldiers during Vietnam days in a new light. Wartime itself yields a new culture and a new set of values.

The clearest signs that attitudes are changing come in the campaign debate. Election 2004 is yielding a surprise. Everyone expected that the experience of Vietnam would influence the discussion of Iraq policy. Instead it is turning out to be the other way around.

© Copyright 2004 Financial Times