By virtue of being on Mars, John Stilson is the last survivor of the human race after the ultimate war, but the Martians have a plan to change all that by sending Stilson back to alter the amount of fissionable material in Earth’s crust.
Wherever in history a decision involving alternatives has to be made, separate and distinct futures branch off, rooted in that choice. There is a world in which the American colonies became a nation, and a world in which they remained under British rule. There is a world in which Franklin Roosevelt was four times elected President, and a world in which the assassination attempt against him in Miami was successful. There is no “might have been,” for the events that “might have been” have actually taken place, somewhere in time—not before, not after, but beside their alternatives.. . .

Variants

(1)
  1. “An Ounce of Prevention,” as by Philip Carter, Magazine of Fantasy and Science Fiction, Summer 1950.
    2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . written by Paul A. Carter as by Philip Carter