Don't watch Threads, OP. It's the British version of 'The Day After' and it's much, much more depressing and dire. If you're curious, here's that film's 'nuke' scene:
As for the OP question, the thing about nukes is that their very existence makes the possibility of usage way too high. All it really requires is one crazed leader with nothing to lose and enough of a chip on their shoulder. Either that or an escalation of conflict that makes their use almost inevitable.
Even then I'll echo the same points being made in that it's much more likely that a rogue party will use one than a country.