The concept of Mutually Assured Destruction (or M.A.D.) widely discouraged the use of nuclear weapons during the Cold War. As both the Soviet Union and the US expanded their nuclear arsenals and capabilities (such as the invention of ICBMs in the late 50s and early 60s), it became clear that if either one were to launch a nuclear missile at the other, it would be met with an equally devastating retaliatory strike thereby leading to the decimation of both countries.
This is why the efforts of the Reagan Administration to create a Strategic Defense Initiative (dubbed Star Wars) that could effectively block a retaliatory strike were met with fear on the part of the Soviet Union: if the US could block the Soviet Missiles, then the Mutual piece of Mutually Assured Destruction would not longer stand thereby putting the Soviets at a distinct strategic disadvantage.
