Nuclear weapons likely preclude large-scale direct conflict between great powers, but they raise the stakes a lot for civil wars, civil disorder, and anarchy within great powers. MAD works great up until a splinter group gains control of a launch facility and says "What, you're gonna nuke San Francisco if we nuke Beijing? Be my guest. We hate those hippie libtards anyway."
Also a point, lots of tricky possibilities in there. I do wonder what would happen if a frustrated national government of one of the great powers tried to use nuclear weapons to subdue a rebellious province. If Russia nuked Ukraine or Poland to keep them in check, would we do anything? If California or Texas was openly defying the Feds and got nuked, would Russia or China do anything?
