Permanently achieved MAD is a myth. The US has been developing its defenses, becoming increasingly capable of intercepting the delivery systems responsible for

Permanently achieved MAD is a myth. The US has been developing its defenses, becoming increasingly capable of intercepting the delivery systems responsible for MAD. Its enemies are responding by developing more nuclear warheads delivered by more and faster missiles. This will keep going back and forth, it must if the balance is to be preserved. The moment one side gets a substantial advantage over everyone else, MAD is no longer assured. If you just build some nuclear weapons and consider yourself in the MAD club because you've checked the "nuclear" box, then cease all relevant R&D and your nation goes permanently out to lunch afterwards, it's not going to go well in the long run unless every other nation does the same (they won't).