Arms Race
During the Cold War the Soviet Union and the U.S. became engaged in a nuclear arms race. Meaning to say they were racing over who could get the most bombs first. At the beginning the US started out very strong easily leading because it had developed nuclear capability first but by 1975 USSR had surpassed the US in number of warheads but two decades later both countries would start to reduce the amount of warheads they had.
However in order to get these warheads to their destination they would need to be sent their either by plane or by missile. The latter would be chosen do to increased capability of ground to air missiles capable of destroying the plane so with missiles they needed to be capable of being sent across the world. And thus an arms race to build ICBM's capable of hitting the other country were developed. However because of M.A.D. the destruction of both attacking countries would be guaranteed which later in the century would result in the decline of nuclear war heads as well as ICBM's and other missiles used to carry nuclear warheads.