THESE days it is only the world’s grandparents who can tell you where they were when they heard John Fitzgerald Kennedy was dead. For decades that was a staple of the global collective memory, a question that could be asked in Berlin or London as readily as New York or Los Angeles. Now that memory has become exactly 50 years old.

Despite its age it is going strong. While some presidents, including those who occupied the White House for a full eight years, have struggled to be remembered at all 50 years after their death, Kennedy continues to loom large. His 1,036 days as president have been the subject of an unending stream of words — filling 40,000 books by one estimate, as well as countless documentaries, TV dramas and Hollywood movies. Like much else of this vast output the latest film, Parkland, focuses on the very last of those thousand-odd days: 22 November 1963.

Interest in JFK peaks for an anniversary, especially a big one. But the truth is, it hardly ever wanes. The Kennedy aura remains a factor in US politics, even when there is no Kennedy on the ballot paper. The standout moment of the 1988 campaign? When Lloyd Bentsen squashed a callow Dan Quayle by telling him: “Senator, you’re no Jack Kennedy.”

One of the lasting images of the 1992 race? Archive footage of JFK shaking the hand of a 16-year-old Bill Clinton at the White House, cherished as if Camelot had witnessed King Arthur anointing a new prince.

There was similar symbolism — the passing of the torch — when in early 2008 Barack Obama won the endorsement of Kennedy’s brother, Teddy, and daughter, Caroline: once he had their blessing, Obama looked unbeatable.

What explains this enduring grip on both the public and political imagination? The manner of Kennedy’s death is central to any answer. The story of the 1963 assassination is so compelling, so full of human drama and pathos and, to this day, mystery — even those who accept that Lee Harvey Oswald was the sole killer cannot agree on his true motive — that it refuses to rest.

But the greater significance of that day in Dallas, beyond the arguments about the grassy knoll and the Zapruder film, is the effect the killing had on how the Kennedy presidency would be viewed thereafter. It would, forever, be a story of what might have been, of potential snuffed out before its time.

As David Ormsby-Gore, then Britain’s ambassador to Washington, wrote to Jackie Kennedy: “He had great things to do and he would have done them.”

Historians do something unusual when confronted with the 35th American president: they debate his actual record less than his potential record.

Take the Vietnam war, the shadow that would hang over the 1960s, thwarting its attempts to be the decade of peace and love. US involvement in that war escalated on JFK’s watch; by November 1963 the number of US troops in Vietnam had risen to 16,000. It was Kennedy, say his critics, who set the course his successor, Lyndon Johnson, would follow by increasing the US military presence to 480,000 during the next four years. After all, Johnson was surrounded by Kennedy’s advisers and insisted he was merely continuing Kennedy policy.

By contrast, JFK’s defenders insist the US president had been a sceptic about the use of ground troops in Vietnam, that he distrusted gung-ho voices in the military, and would have found a way to wrench the US out of that quagmire.

On civil rights, that other defining struggle of the 1960s, the argument is equally divided. Admirers cite Kennedy’s televised address to the nation, referring to the battle over racial segregation as a moral crisis, and his readiness to use the national guard to force the whites-only universities of the south to open up to black students.

Those less enamoured say he was late to the issue and that he was unlikely to have been willing or able to ram through the landmark civil rights legislation eventually passed by Johnson.

“Kennedy had no great understanding of the impatience of African Americans or the intransigence of white southerners, while Johnson — from Texas — understood both,” says Tony Badger, professor of American history at Cambridge. President Kennedy, says Badger, was more “scared of the south” than LBJ, adding the reminder that Jackie Kennedy referred to Martin Luther King as that “terrible” man.

There’s a similar debate over Johnson’s “war on poverty”, with Kennedy’s advocates insisting that everything LBJ did JFK would have done too, if he only had had the chance.

More clear-cut, and usually held up as the unambiguously golden part of Kennedy’s legacy, is his handling of the Cuban missile crisis of 1962. As those over 60 cannot forget, the world held its breath for those few days, genuinely believing the standoff between Washington and Moscow over the Soviet deployment of nuclear weapons in Cuba could end in Armageddon.

Kennedy was cool-headed, faced down Washington’s hawks and showed great creativity, and even empathy, in his dealings with Nikita Khrushchev. In pulling back from the abyss JFK secured his place in history (and laid the ground for the nuclear test ban treaty signed weeks before his death). Knowing his standing would never be higher, he turned to Jackie when the crisis was finally resolved and said: “Well, if anyone’s ever going to shoot me, this would be the day they should do it.”

RARE POLITICIAN

Yet judging Kennedy by this standard, assessing his policy failures and successes, is slightly to miss the point. His appeal, and the enduring power of his memory, lies elsewhere.

For one thing, he was that rare politician able to inspire. The young especially responded to his call, “ask not what your country can do for you ... “, while his declaration in a city divided by the cold war that “Ich bin ein Berliner” resonated throughout east Europe. Less than three years in office, Kennedy nevertheless conjured up oratory and imagery that retain their hold half a century later.

There’s no use pretending that sex and glamour were not at the heart of this. John Kennedy looked young, vigorous and handsome, and had a beautiful wife to match. Stark was the contrast with both his predecessors in the Oval Office and his counterparts abroad: how different Kennedy looked from Macmillan, Adenauer and De Gaulle. He appeared like a new leader for a new era.

That image has endured far beyond the archive footage. Kennedy established a template for political leadership that is still in place, in the US and around the world. Kennedyesque is still the style, the demeanour, that candidates for high office aspire to: slim, energetic, accompanied by a supremely elegant spouse. Whether it’s Obama in Ray-Bans or Cameron on the beach with Samantha, JFK remains the model.

Of course, much of it was fake. Unknown to the voting public, their fit young president was, in fact, crippled with back pain from Addison’s disease, taking industrial quantities of drugs to get by. Equally concealed were his serial infidelities, affairs with women ranging from 19-year-old interns to Marilyn Monroe — a record of womanising inside the White House that makes Clinton look like a boy scout. Yet none of this seems to diminish the legend. For JFK, the first president of the TV age who understood and exploited the medium, remains still a celebrity. He is the hero of a story that has everything: sex, lies and 8mm film, gossip, intrigue and lust — all set against a background of war and peace.

And all that is combined with something perhaps as powerful as sex: hope. Despite everything, the Kennedy brand still stands for idealism, for the ambition of a moon landing and the call to public service enshrined in the Peace Corps.

Celebrity and hope: it’s a powerful, quintessentially American, combination. Fifty years ago the man who embodied it was gunned down. But the myth lives on. Not even a magic bullet can destroy that.

By arrangement with The Guardian