There's only a theoretical chance of a nucelar weapon usage. It's like two men stuck in a closed room, each holding a hand grenade. Regardless who throws it first, both are dead.
This is why no nation would launch their nuclear missiles. The moment you fire your missile towards my capital, regardless how many lives you've destroyed, I'll be capable to order my own army to do the same to you. So it's pretty much a no-win situation.
This is why all the nuclear missile silos are placed in remote locations. Not like they're in the cities so if you take out a city, you've taken out the missiles as well.
With the technology we have these days, we're perfectly capable of mass killing ourselves with a good old conventional weapons. Just drop a CBU class bomb over a populated area or a battlefied and that's it..