What is being defended exactly? What makes it worth the risk of destroying the planet and everyone on it? If someone wants to say they are the boss of me that badly, they can have it as far as I am concerned. It is the dumbest, most self-defeating game of bluff and counter-bluff I can imagine. You are talking about two people pointing loaded guns at each other, and betting neither will fire because the other might also get a shot off before they die. What the hell kind of way is that to exist? If the 'enemy' wants to rule the world that badly, he can have it. Who cares who gets to call himself the winner? Better that than no world at all.
What you have described is the nuclear deterrent. USSR and USA can initiate that, kill the other side, and they will kill our side, that is why it never happened. Arch enemies, but not stupid. ISIS, Pakistan/India, North Korea, not as smart. The only bonus is northern hemisphere.