Mutually Assured Destruction (MAD) is a Cold War-era military theory that was developed to deter the use of nuclear weapons. The theory is based on the idea that nuclear weapons are so destructive that neither side would use them because both sides would be totally destroyed in the conflict. This fear of mutually assured destruction helped prevent the Cold War from turning into a hot war.
After the end of World War II, the Truman administration was initially undecided about the utility of nuclear weapons. It considered them as weapons of terror rather than part of a conventional military arsenal. The bombings of Hiroshima and Nagasaki made nuclear weapons unusable.
In the 1950s, the Eisenhower administration's policy was to maintain an imbalance of terror in the West's favor. The U.S. increased it stockpile of nuclear weapons to 18,000 by 1961. U.S. war plans featured nuclear overkill, meaning that the US could launch an excessive nuclear attack more than the Soviets could. Eisenhower and the National Security Council believed that preemption—the launching of an unprovoked attack—was a military option.
The Cuban missile crisis drove President Kennedy and then Johnson to develop a "flexible response" to replace the pre-planned overkill. By 1964, it became clear that a disarming first strike was increasingly infeasible.
The MAD strategy was developed. The US, USSR, and their allies possessed nuclear weapons that could completely destroy the other side. They threatened to do so if attacked. The missile bases located abroad became a source of friction, because the host nations also faced destruction.
Supporters of MAD argued that the fear of mutually assured destruction was the best way to secure peace. They believed that this fear would prevent any leader from using nuclear weapons because they would be destroyed in the conflict. Alternatives such as attempting a limited nuclear exchange or developing an effective first strike capability were rejected because they could tempt some leaders to act.
Opponents of MAD, on the other hand, argued that MAD was based on fear and cynicism. It led both sides to building more destructive bombs and more sophisticated ways of delivering them.
Overall, Mutually Assured Destruction strategy was based on the idea of the risk of mutually assured destruction, by which each side would assure the other that any attack would be met by an overwhelming retaliation. MAD became the central principle of US and Soviet defense planning during the Cold War.
Source: What Is Mutually Assured Destruction?
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