The assassination of John F Kennedy was a catastrophe in itself, but the consequences could have proved far bloodier than any event since World War II.
The assassination of the 35th President of the United States 40 years ago in Dallas, Texas, came at a particularly tense point in the Cold War.
There is no evidence of anyone beside Oswald being involved
Kennedy's successor Lyndon Johnson immediately realised that if - as many suspected - the Cubans or the Russians had conspired to kill Kennedy then the result would be a nuclear war.
It could have cost 40 million American lives, plus untold numbers of Soviet, Cuban and European casualties.
Conspiracy here, conspiracy there
There is no doubt that the Kennedy assassination and the conspiracy theories surrounding it continue to fascinate ordinary Americans and people all over the world.
Like most journalists who have travelled extensively in the United States, I have been assured by various Americans at various times that JFK was indeed murdered by the Russians, or by the Cubans, or by the CIA and the FBI, or by all of the above acting together.
Then of course, there is the spectacularly nutty theory of the film-maker Oliver Stone that Kennedy was really murdered by a secretive right-wing cabal in order to put Lyndon Johnson into power.
I would love, of course, to be able to prove that some or indeed all of the conspiracy stories are true.
But, as we report in The Kennedy Assassination: Beyond Conspiracy there is no factual basis to believe any of the conspiracy theories at all.
The truth about what happened that bleak November day in Dallas is simpler, yet no less fascinating.
For the first time, drawing on extraordinary computerised reconstructions based on a home movie shot at the assassination scene by a Dallas dressmaker, Abraham Zapruder, the conspiracies can, in my view, finally be laid to rest - completely and forever.
Here is what we know for sure: John F Kennedy was murdered as his motorcade drew through Dallas in November 1963.
He was in an open-topped limousine. He was assassinated by Lee Harvey Oswald who worked in the Texas Book Depository overlooking the assassination scene.
Oswald's fingerprints are all over the area from where the shots were fired. His rifle did the shooting. He was the only employee of the Book Depository to go missing.
He then murdered a Dallas cop who tried to talk to him, and finally he himself was arrested in a movie theatre.
All this would seem to point to an open and shut case.
But here is the problem. Oswald was an ex-Marine sharpshooter who had learned Russian and tried to defect to Russia.
He had a Russian wife and called himself a Communist.
The Texas Book Depository overlooked the scene
He agitated on behalf of Castro's Cuba.
And Vice-President Lyndon Johnson had been told that if the Soviet Union ever planned to attack America, it would probably begin by trying to murder the president.
Then Oswald himself was murdered - on national television - by a Dallas strip-club owner, Jack Ruby, who allegedly had ties to the mafia.
Plus Kennedy's brother Robert was in charge of a top secret programme to assassinate Fidel Castro, as well as cracking down on the mafia.
Start putting it together and you can easily see where the conspiracies theories began.
The Mob, the Cubans, and the Russians all had reason to want Kennedy dead.
The idea that Lee Harvey Oswald, an under-achieving awkward misfit, acting alone could murder the most powerful man in the world is simply an example of how banal murders can be.
The absence of any real evidence of involvement by anyone other than Oswald, plus the computerised reconstructions of the assassination, which demonstrate how a single bullet passed through Kennedy before injuring Texas Governor John Connally in the front seat of the limousine, convinced me that there was no conspiracy.
Oswald fired the fatal shot.
There was no second gunman.
There was no conspiracy.
The account of Oswald's tortured personality by his brother and those who knew him points to a loner desperate to secure some kind of fame in a world which rejected him.
Nowadays he would probably try to get on Big Brother.
In November 1963, Lee Harvey Oswald sought his 15 minutes of fame by shooting dead the president of the United States.
We are still coming to terms with the consequences, and - despite the compelling evidence in our film - I suspect it will be a long time before the conspiracy theorists are finally silenced.
The Kennedy Assassination: Beyond Conspiracy was originally broadcast on 23 November, 2003 and will be repeated on Wednesday, 1 September, 2004 on BBC Two at 2100 BST.