Technically that goes in Sci-Fi, lol
actually if you read the history of Fallout, it is set in an Alternate history of our current history. the universe spits right after WW2, this is just in the future of said alternate history.
The divergence take place not right after WWII but in times of Cold War. There is no clear moment or moments that lead to this, no single situation or case. This is a forgoten/not written by now element of the lore.