The strategic doctrine that nuclear war can be prevented by ensuring that any nuclear attack would trigger a retaliatory strike capable of inflicting unacceptable damage on the attacker — making the initiation of nuclear war irrational for any state, and thereby achieving peace through the permanent mutual threat of civilizational destruction.
Nuclear deterrence is the most consequential idea in the history of strategy, and it rests on a premise that no one can test without destroying civilization: that rational leaders, facing the certainty of their own destruction, will refrain from initiating nuclear war. The theory is internally coherent. The historical record suggests that deterrence has, so far, held — no nuclear weapon has been used in war since 1945. But the historical record also shows that deterrence has survived several occasions when it almost failed, through a combination of institutional procedures, individual judgment, and luck that the theory did not predict and cannot guarantee.