In this age of a sole and overweening superpower, a younger generation will find it impossible to believe. But just 25 years ago the USA was on its knees, its presidency disgraced, its allies falling prey to Communism. Desperate crowds clawing to attach themselves to the final helicopter out of Dha Trang, the last marines humiliatingly plucked from the roof of the US embassy in Saigon as a mob prepared to sack the building - the images are true, but today seem like a dream. A quarter of a century ago, however, Vietnam was a reality that none of us, even non-Americans, could escape.
Few who were young at the time will forget the violence in front of the US embassy in Grosvenor Square in 1968. Today's anti-capitalist demonstrations against the World Trade Organisation and global capitalism's iniquities pale beside the fury and the frustration catalysed by Vietnam, a crusade against the US and all the evils of racism and imperialism, consumerism and war in general.
The anti-Vietnam protests, fuelled to some extent by the fact that this was the first war that everybody could watch unfold on the television screens in their living rooms, spanned the Atlantic.
It was the sight of protesters outside the White House in their tens of thousands that helped to persuade the then President Richard Nixon that he had to find a solution that would extricate the US from the war with as much honour as possible.
Students across Europe took up the anti-war, anti-imperialist cause. There has been nothing to compare with this world-wide surge of protest, which at the very least was a factor in changing the policy of a world superpower. The anti-Communist demonstrations that swept through Eastern Europe in 1989 came from the other side of the political spectrum.
All of us who lived through the war have our own memories. Vietnam brushed me twice, first personally, and then as a tiny fragment of history. The first occasion was in 1970, when Alec Hottell, a West Point graduate and Rhodes-scholar friend and contemporary at Oxford, was killed on service there in a helicopter crash.
The second was an early February day, decades later, in Washington, when news broke about the manoeuvrings of a far more famous Rhodes scholar to avoid the Vietnam draft.
The Wall Street Journal published its story in the midst of the extraordinary campaign for the 1992 New Hampshire primary, when candidate Bill Clinton was already struggling to cope with the revelation of his affair with the night-club singer Gennifer Flowers. That same evening I had been invited to dinner by yet another former Rhodes scholar, Strobe Talbott, who became a close friend of Clinton's when they shared digs in north Oxford in the late 1960s, and today is his deputy Secretary of State.
On the late news we watched the Governor of Arkansas trying to explain his behaviour: not least why, at the age of 22, he was already agonising about how to preserve his "future political viability" if he failed to serve in Vietnam. Strobe was depressed, sure that this second blow would finish Clinton's candidacy. Almost 17 years after its end, the war, it seemed, was still a bitter enough memory to deprive American politics of someone he believed could become a truly great president.
Talbott wasn't alone. Every talk-show pundit in the land declared Clinton finished. Americans might be more relaxed about extramarital adventures by their politicians, it was held, but they still regarded attempts to wriggle out of military service as deeply unpatriotic. Clinton, to borrow their charming phrase, was "dead meat".
We were all wrong. One reason was Clinton's superhuman resilience as a candidate. But there was another, more profound factor. Somehow, Vietnam no longer mattered. It had been a war of its time, born, like most conflicts, from a mixture of pragmatism and idealism, but very much a product of the global struggle with the Soviet Union. It was fought in the ostensibly impeccable cause of halting Communism, of stopping dominoes in South-east Asia from falling, and preserving "freedom".
Vietnam was an unmitigated disaster, the only war the US has ever lost. It took the lives of 58,000 Americans and an estimated 2.5 million Vietnamese. It cost untold treasure, destroyed a president, and fired the protest of a generation at home and around the world as no event since. And for what? Vietnam fell, but the dominoes didn't. Today, Communism survives in a few pockets such as Cuba, North Korea and Vietnam itself, but the American way has triumphed.
Clinton's survival and ultimate victory in 1992 was a first sign that the US was getting Vietnam out of its system. In a sense, the 2000 campaign has rounded off the process. With his candidacy, John McCain, a US Navy pilot in the war and a prisoner in Vietnam for five-and-a-half years, sealed the rehabilitation of the war's last outcasts, the soldiers who had returned, only to be shunned and forgotten by an ungrateful country.
But McCain was in the end soundly beaten for the Republican nomination by George Bush Jr, whose record in Vietnam consisted of sitting it out in the Texas National Guard. This year's crucial primary in South Carolina was instructive. The state was brimming with Republican-voting war veterans, yet McCain's compelling record as a military hero was no match for the hostility of the religious right, whom he had grievously offended.
As for the US's early baby-boomers of my own generation, who variously accepted, manipulated or dodged the draft, many are now only a few years short of retirement, ensconced in a prosperity generated by the longest boom since the Second World War. Vietnam is history - history in some respects more remote than the Korean war that predated it. Handling North Korea is still a daily torment for US policy-makers. But there is no ball and chain attached to the US's collective political psyche marked "the Legacy of Vietnam".
If such a shackle exists, it is in the Pentagon. The new doctrine, exemplified by the war against Saddam Hussein, has three parts: the use of overwhelming force, no body bags and no "mission creep". Never again will the US allow itself to be sucked into an open-ended and unwinnable foreign war, in which American boys die even when the country's national security is not directly at stake.
The real world naturally does not respect such paradigms. You can send cruise missiles smashing into targets in Afghanistan and Sudan - but "mission creep" is precisely what confronts the thousands of US troops stationed in Bosnia and Kosovo, and likely to remain there for many years to come. But as long as the body-bag count stays at zero, the post-Vietnam theory holds.
Vietnam remains a one-party Communist state, one of the 25 poorest countries in the world. An economic deal offered by the US is on ice. But Clinton presses on. He lifted the US trade embargo in 1994 and established diplomatic relations in 1995. Later this year he may become the first US president to set foot on Vietnamese soil since 1969, when Richard Nixon dropped by to give the troops a pep talk. The "Legacy of Vietnam" may yet end up as part of the legacy of Bill Clinton.