The idea that the threat of mutual annihilation prevents nuclear war. If attacking guarantees your own destruction, no rational actor pulls the trigger.
This only works if both parties have nukes, currently 9 countries have nukes while the majority 185 countries don’t. This asymmetry stands against the doctrine of nuclear deterrence.
Those who have nukes are the only ones who have a real say in this world, the vast majority of the countries without nukes are completely at the mercy of a small handful of countries with nukes.