We all know that the Fallout universe diverged from our own some time ago...but when? It's certainly a matter of debate, but what we do know is that divergence occured some time between the end of World War II in 1945 and the
However, the gap could be even shorter. In October 1st 1949 in the real and Fallout world, the People's Republic of China was founded. Also, many songs released in the mid-fifties (to about 1955) are present on radio stations in Fallout 3 and New Vegas, such as Let's Go Sunning, which was released in 1954. Also, in the Fallout world there was no NASA, instead there was the USSA (United States Space Administration). This means that the National Aeronautics and Space Act of 1958 was never signed (or at least caused the creation of the USSA instead of NASA). This means that divergence had to have occured some time before 1958.
However, this is only following something I've dubbed the limited divergence (LD) theory, meaning divergence occured between the last event shared by both universes and the next event shared by the Fallout universe. For example, this would mean that it happened between at least 1945 and at the latest 1958.
However, there's another theory: there could have never been a 'real' universe at all: perhaps the universe could have always been the Fallout one (as evidenced by its own nuclear physics, e.g. ghouls) but events unique to it may have only started regularly occuring around 1945. What I mean is that in this universe there have always been fictional events (such as the founding of Sunset Sarsaparilla in 1918) but fictional events only began to distort the timeline at a certain year after World War II ended. I've called this is the unlimited divergence (UD) theory.
I believe that limited divergence is the most likely theory: every fictional universe has its own fictional events that happened before the present, and Fallout is no exception. In my opinion, divergence occured between 1955 and 1958. Thanks for reading! :)