Definition: Military theory of nuclear deterrence holding that neither side will attack the other if both sides are guaranteed to be totally destroyed in the conflict. The theory was developed during the Cold War, when the US, USSR and respective allies held nuclear weapons of such number and strength that they were capable of destroying the other side completely and threatened to do so if attacked. Proponents argued that the fear of MAD was the best way to secure peace, rather than threatening a limited nuclear exchange from which one side might hope to survive with an advantage, which might tempt some leaders. For long periods of the Cold War MAD entailed a relative lack of missile defenses so as to guarantee mutual destruction.
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The fear of Mutually Assured Destruction helped prevent direct conflict.
