We wouldn't. That's the whole point. But then no one would attack us if they knew that we would retaliate. JC was pushed on it, and would not confirm that he would. Surely that is the whole point of a nuclear deterrent. To deter!
And what would a country achieve from preemptively attacking us with nuclear weapons? As I've already said it wouldn't be a precursor to an invasion, so it would only be to annihilate us. Where do you think that would leave the aggressor on the world stage? Is that not a deterrent in itself?