Basically what Kit said about the theory of mutual destruction.
But honest to god, I have a different theory about a country with nukes vs. one that doesn't.
I am just curious if say the United States nuked the shit out of a non-nuclear nation, would anyone else pull out their nukes? Would Russia pull out their nuclear weapons on us? I am more inclined to saying they know that if they do that, we will do the same, causing both of us to die. Nobody wants to sign their own countries notice to be exterminated. They would be pissed off at them, but I doubt they'd nuke them.