What if John F Kennedy had returned home safely from Dallas on 22 November, 1963? Would Congress have passed the Civil Rights Bill? Would the South have defected to the Republican Party? Would millions have died in the conflict in Vietnam?
History is not just a litany of all the things that happen, it is about what did not happen, about the road not taken.
And sadly that history, the history of things that did not happen, is unknowable. Which is what makes it tantalising and fascinating.
But there is a way to catch a glimpse of that history, to see, or maybe just to feel what it might have been like. And it lies in the archives. In the notes for speeches written but never given.
When America sent men to the Moon it was more confident it could get them there than it was that it could get them back.
So William Safire, Richard Nixon's brilliant speechwriter, persuaded his White House colleagues of the need for a speech 'In Event of Moon Disaster'. The draft is a brilliant, poignant, piece of writing, even read with the comfort of knowing that it wasn't needed.
"These brave men, Neil Armstrong and Edwin Aldrin, know that there is no hope for their recovery. But they also know that there is hope for mankind in their sacrifice.
"These two men are laying down their lives in mankind's most noble goal: the search for truth and understanding.
"They will be mourned by their families and friends; they will be mourned by their nation; they will be mourned by the people of the world; they will be mourned by a Mother Earth that dared send two of her sons into the unknown.
"In their exploration, they stirred the people of the world to feel as one; in their sacrifice, they bind more tightly the brotherhood of man.
"In ancient days, men looked at stars and saw their heroes in the constellations. In modern times, we do much the same, but our heroes are epic men of flesh and blood.
"Others will follow, and surely find their way home. Man's search will not be denied. But these men were the first, and they will remain the foremost in our hearts.
"For every human being who looks up at the moon in the nights to come will know that there is some corner of another world that is forever mankind."
Two of Nixon's predecessors have speeches on file that told tales of even greater disasters that never happened.
General Eisenhower, for instance, had words ready in case the D-Day landings failed. He took the blame upon himself: "If any blame or fault attaches to the attempt it is mine alone."
Even more alarming is to learn of the preparations John F Kennedy made "with a heavy heart, and in necessary fulfilment of my oath of office".
This was to announce that military operations had begun to remove nuclear weapons from Cuba. We now know enough to be reasonably confident that such action would have led to nuclear war.
Most of the time speechwriting is a more prosaic endeavour, writing for moments that are humdrum. Such a moment was the routine speech John F Kennedy planned to deliver one day in Dallas, Texas, at the Trade Mart.
"We in this country, in this generation, are - by destiny rather than choice - the watchmen on the walls of world freedom. We ask, therefore, that we may be worthy of our power and responsibility, that we may exercise our strength with wisdom and restraint, and that we may achieve in our time and for all time the ancient vision of 'peace on earth, good will toward men'.
"That must always be our goal, and the righteousness of our cause must always underlie our strength. For as was written long ago: "Except the Lord keep the city, the watchman waketh but in vain."
Speech writing at its best is highly creative, but much of the time, between creator and delivery bureaucracy intervenes. Or accident.
Working overnight on William Hague's first party conference speech someone stepped on the printer cable and a page on crime disappeared.
The next day some liberal journalists congratulated the speech team (me, among others) on omitting the usual Tory conference crime stuff. But speeches on the cutting room floor tell a story.
And there are few moments where that is more obvious than this week, when David Miliband announced that he was standing down from Parliament. If he had won there were to have been words, a leader's speech never given.
And as he leaves it is just those words, that little hint of what might have been that he leaves behind.
The hint of a future that became a past without being a present.
Archive on 4 - In Event of Moon Disaster - is on BBC Radio 4 on Saturday 30 March at 20:00 GMT