$1:
What's the world's best defence against nuclear weapons?
There isn't.
All the blah blah blah from the 'leaders' hasn't done anything to stop the Norks,
or the Iranians.
And all the blah blah blah won't stop anyone else from getting their hands
on that kind of materiel.
Building a bomb isn't that difficult.
What is difficult is building them small enough to put it into something that can't
be intercepted and shot down, i.e. a missile.
Everyone has seen other countries build up nuclear weapons, and they
will try it themselves one day.
The real problem is not going to be a missile strike from Islamic State;
it will be a nuke in a truck bomb or a ship.
Good luck with that.