To avoid invasion a country must have a nuclear arsenal of sufficient size to initiate total annihilation. At that very point the country becomes in invader and not a defender. It is how that capability can be misused by shithead leadership that presents the problem. I don't know how that problem can be solved.

The good news is that any leader, shithead or not, who would actually make the threat of total destruction true, would have to be totally bat shit crazy.

Oh wait..

Shit