Why do people remember where they were and what they were doing the day President Kennedy died? Did he really have that much influence and charisma? In historian Robert Dallek's living room, a bronze bust of John F Kennedy stands in a place of pride, his expression fixed in metal, youthful and handsome. Yet had he lived beyond that day in Dallas, his statue might have borne a different countenance, perhaps wrinkled and careworn.
It is impossible to divine what the 46-year-old first-term president would have become, just as it is impossible not to try to imagine what America would have been like if he had lived. The title of Professor Dallek's first Kennedy book was 'An Unfinished Life', which begs that very question ; what Kennedy could have become and what he stands for now.
"It's the fact that people have been so disappointed in subsequent presidents," he said. "Lyndon Johnson, the failure in Vietnam. Richard Nixon having to resign over the scandal about Watergate. Gerald Ford's truncated presidency. Jimmy Carter is seen as a failed president."
"The two Bushes, one loses after a term, the second leaves under a cloud because of no weapons of mass destruction in the Iraq War, because of Katrina - the disaster in New Orleans - because of the economic downturn," he added.
"People want a better life in this country. They want to think their children are going to do better. And they associate this with Kennedy's youth and his promise. He died at the age of 46 - he's a blank slate....no sins to complain about and endless possibilities."
Professor Dallek's latest book 'Camelot's Court' focuses on JFK's advisers, and in particular their influence on foreign policy. One of the biggest "what ifs..." is Vietnam.
"There are many historians and others who will say that Lyndon Johnson's war - which ended in such disaster - had warning preludes in the Kennedy administration," he said.
"Johnson really had no choice, it was continuity. The other side of this, however, is that Kennedy was under tremendous pressure from various advisers to increase American involvement in that war during his 1,000 days," he added. "People pressed him to put ground forces in and he did not want to do this. We'll never know what Kennedy would have done in Vietnam. I don't think he knew."
"I don't think he ever would've put 545,000 troops into Vietnam," he said. "Indeed, after the Bay of Pigs operation and the Cuban Missile Crisis, there was pressure on Kennedy from his military to consider thinking about invading Cuba. They told him Khrushchev may be hiding the missiles in caves. 'You're still going to have to go in there'.
"So they make contingency plans. Robert McNamara, the secretary of defence, puts an invasion plan before him and Kennedy writes McNamara. 'Bob', he said, 'We have to remember what happened to the Russians in the Winter War with Finland, and what happened to us in Korea. We could get bogged down'," he added.
"Now this is him talking about an invasion of Cuba. So you could imagine the way he thought about Vietnam."
Dallek thinks conspiracy theories still abound because people are unwilling to admit that luck - bad luck - can have such an impact on a nation's destiny.
"I don't think the country's gotten over his assassination yet, in part because it's a terrible blow to the country's self-esteem. The feeling is this is not what we do in American politics," he said. "This is not a banana republic, we don't have coups d'état, we don't topple governments and kill our leaders. So to this day, 59% of Americans believe there was a conspiracy because they have to believe there was something larger at work here. "
"Someone as inconsequential as Oswald could not have killed someone as consequential as the president. That's what I think many people feel. And it couldn't have been that fortuitous," he added. "How could it be that this ne'er-do-well, Oswald, getting off those shots - was he such a splendid marksman? What's missed in this is how fortuitous the world can be."
"Indeed, the first shot that struck Kennedy passed through his neck. He was wearing a back brace, which he always did at these public events to deal with the terrible back pain he had," he continued. "If he hadn't had the back brace on, that bullet would've toppled him, knocked him to the side and the bullet that then found the back of his head and killed him never would have found its mark."
"He is the most popular president in American history, currently. There was a poll a few years ago asking people to assess the last nine presidents from Kennedy to George W Bush - he had an 85% approval rating," he said. "The only one within hailing distance of him was Ronald Reagan with 74%. So he has this phenomenal iconic hold. "
"The interesting question to me is, 50 years from now, on the 100th anniversary of his assassination, will we still be talking about him? You and I won't be sitting here doing it," he laughs. "But will others be talking about him? I think it depends a lot on what happens in the future, showing you how historical reputation is very much shaped by current and future events."
"If we get another president with Kennedy's kind of star quality and command of public approval, then he/she might eclipse Kennedy but so far nobody qualifies," he added. "If we continue to stall and struggle into the future, it may well be that Kennedy will go on having this phenomenal hold on the public's hopes and dreams and imagination."