The answer is "the idea that the threat of nuclear war was enough to stop an attack".
Deterrence theory holds that nuclear weapons are planned to stop different states from assaulting with their atomic weapons, through the guarantee of countering and conceivably mutually assured destruction (MAD).
The idea of nuclear deterrence pursues the basis of the 'first user' standard: states maintain whatever authority is needed to utilize atomic weapons in self-protection against an outfitted assault undermining their crucial security interests.