Creating a nuclear explosive is more of a routine engineering challenge, rather than intellectual. Any nation with (1) 1950s-level engineering capability and (2) money or will can do it.

To badly paraphrase a Manhattan Project scientist, from memory: "there are lots of ways to create a nuclear explosion and no matter which one we try they all seem to work"

Miniaturised nukes are much, much more complicated and seem to require 1990s+ levels of physics, computer modelling and engineering.