This is probably Isaac Asimov's "Spell My Name with an "S"".
The story concerns Marshall Zebatinsky, a Polish-American nuclear physicist. He is concerned that his career has stalled, and in desperation consults a numerologist for advice on restarting it. The numerologist advises him to change the first letter of his name to "S": Sebatinsky.
A complicated series of events ensue in which Sebatinsky is investigated by the Security establishment, who feel that he must be trying to hide something. His Polish origins lead them to suspect that he is trying to distract attention from relatives in the Eastern Bloc. They discover that he does have a distant cousin who is also a physicist, which leads to the discovery that the Soviet Union is working on force-fields as a nuclear defense.
The Americans immediately start to develop their own counter-defense. They still have no real reason to suspect Sebatinsky, but just in case they look for a discreet way to get him out of the classified project in which he is engaged. Much to his surprise (and delight), Sebatinsky is appointed to a senior professorial post, which is exactly what he hoped for when he went to the numerologist.
At the end of the story, it is revealed that the numerologist is actually an extraterrestrial and that he was involved in a bet as to whether he could avert a nuclear war on Earth with a minor stimulus (see butterfly effect). Though it is not explicitly specified, it is clear that the extraterrestrial in question is a brilliant school-boy (or the equivalent among creatures of pure energy who live in space) who made a bet with another pupil. The story concludes with the same two beings making another bet to put everything back the way it was with another minuscule alteration, with the inevitable consequence that the world will end in nuclear holocaust.
I found it on the TV Tropes For Want of a Nail Literature page, although I think I'd read it before.