There are so many nuclear devices left over from the excesses of the Cold War that it is not difficult for anyone to get their hands on them. Some are home-made from the tons of fissionable material which resulted from the expansion of the nuclear industry when fossil fuels ran out. Entire capitals vanish instantly in blazes of atomic fire. Some scientific experts point to the moon and the asteroids as possible sources of the materials now so desperately needed on Earth, and wonder about harnessing the sun's energy in space, but to no avail. Even the wealthiest nations can no longer afford space travel.
Social structure breaks down and resource depletion intensifies.
Our technological level regresses to that prior to the industrial revolution, only we are in much worse shape for advancing than we were back then. All of the easily-retrieved resources of coal, oil, metals and timber are gone. As starvation closes in, some folks invent little food wafers called Soylent Green. One of the wonderful things about science fiction is that it shows us possible futures. Some look like nice places to live.