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The Doctrine of Mutual Assured Destruction:
Mutual assured destruction (MAD) is a doctrine of military strategy in which a full-scale use of nuclear weapons by two opposing sides would effectively result in the destruction of both the attacker and the defender. It is based on the theory of deterrence according to which the deployment of strong weapons is essential to threaten the enemy in order to prevent the use of the very same weapons. The strategy is effectively a form of Nash equilibrium, in which both sides are attempting to avoid their worst possible outcome — nuclear annihilation
Theory
MAD is a "Poison Pill" strategy. The doctrine assumes that each side has enough nuclear weaponry to destroy the other side and that either side, if attacked for any reason by the other, would retaliate with equal or greater force. The expected result is an immediate escalation resulting in both combatants' total and assured destruction. It is now generally hypothesized that the nuclear fallout or nuclear winter resulting from a large scale nuclear war would bring about worldwide devastation, though this was not a critical assumption to the theory of MAD.
The doctrine further assumes that neither side will dare to launch a first strike because the other side will launch on warning (also called fail-deadly) or with secondary forces (second strike) resulting in the destruction of both parties. The payoff of this doctrine is expected to be a tense but stable peace.
The primary application of this doctrine started during the Cold War (1940s to 1990s) in which MAD was seen as helping to prevent any direct full-scale conflicts between the United States and the Soviet Union while they engaged in smaller proxy wars around the world. It was also responsible for the arms race, as both nations struggled to keep nuclear parity, or at least retain second-strike capability. Although the Cold War ended in the early 1990s, the doctrine of Mutually Assured Destruction certainly continues to be in force.
Proponents of MAD as part of U.S. and USSR strategic doctrine believed that nuclear war could best be prevented if neither side could expect to survive a full scale nuclear exchange as a functioning state. Since the credibility of the threat is critical to such assurance, each side had to invest substantial capital in their nuclear arsenals even if they were not intended for use. In addition, neither side could be expected or allowed to adequately defend itself against the other's nuclear missiles. This led both to the hardening and diversification of nuclear delivery systems (such as nuclear missile silos, ballistic missile submarines and nuclear bombers kept at fail-safe points) and to the Anti-Ballistic Missile Treaty.
This MAD scenario is often referred to as nuclear deterrence. The term deterrence was first used in this context after World War II; prior to that time, its use was limited to legal terminology.
In practice, the theory proved both utterly effective and exceptionally dangerous (e.g., Cuban Missile Crisis) through the end of the Cold War. Today, all lesser nations are believed to be keenly aware that any use of nuclear weapons, in any context, is the recipe for their annihilation. Significant nuclear powers, such as the United States and the Russian Federation, operate under the deterrent effect of potential retaliation with respect to "first use" in the conduct of brush fire wars and other lesser conflagrations. The U.S., as possessor of the largest and most deployable stockpile of nuclear weapons, continues to exercise its vast nuclear might as a cornerstone of its foreign policy with regard to rogue states and communist nations that currently or may soon possess nuclear weapons technology. U.S. military forces stand on permanent alert in order to deter potential nuclear adversaries. Likewise, non-democratic nations cannot use nuclear weapons against the U.S., or her critical allies (United Kingdom, Canada, Japan, Germany, Israel, Australia, and South Korea) without threat of (as U.S. President John F. Kennedy said) a "full retaliatory" response by the United States.