Our adversaries have understood that for a long time, just like we have. Send a 1 MT a-bomb, up to 100 miles, and detonate it, and anything up there is going to be destroyed.
And a powerful EMP will be hot on the heels of the explosion. But, once any bomb is used, it will be met with retaliation, and that has worked up until now.
With NK, they seem to stupid to understand the concept of MAD, and they may be the ones to bring on a nuke exchange.