And then, what?
Suppose the world agrees to dismantle its nuclear arsenal but a single nation, the great atomic nation of Nuclearia, decides that it will keep its weapons and it will destroy the world unless every other nation obeys its rule. And assume Nuclearia has magickal weapons that do not affect Nuclearia lands, or its citizens. The world refuses to obey and Nuclearia unleashes the nuclear holocaust.
Now what? What did Nuclearia achieve by destroying the rest of the world with nuclear weapons? What will Nuclearia do in a world of its own? Note that the rest of the world is now a radioactive waste where nothing lives and nothing grows. Other nations' lands cannot be annexed and used for farming, because there is no fertile soil left anywhere. While some intrepid souls no doubt long to visit the great glass fields of New York, spending any time outside Nuclearia is deadly and most of the world is a depressing burned desert so travel is pointless and tourism is a joke. International commerce of course is out of the question because there is no other nation than Nuclearia. Any resources, such as metals, gases, fossil fuels etc are limited to what Nuclearia has in its own territory. Any scientific progress is limited to what Nuclearian scientists can achieve on their own, without any input from the outside, given that there is nothing on the outside.
How does destroying everyone else increased Nuclearia's chances of survival?
How do you protect yourself by destroying everyone else?