The Fallout timeline branches off from ours at the point where the transistor was supposed to be invented, which was somewhere between 1945 and 1955. Fallout's "Great War" and the transformation from the US to the wasteland began and ended on the 23rd of October, 2077, caused by conflicts over resources, caused by the Fallout timeline's less energy-efficient technology (due to their lack of transistors). This does create a 100-year gap in which, apparently, zero cultural progress happened.