Deterrence theory refers to scholarship and practice on how threats or limited force by one party can convince another party to refrain from initiating some course of action. The topic gained increased prominence as a military strategy during the Cold War with regard to the use of nuclear weapons and is related to but distinct from the concept of mutual assured destruction, which models the preventative nature of full-scale nuclear attack that would devastate both parties in a nuclear war. The central problem of deterrence revolves around how to credibly threaten military action or nuclear attack despite its costs.
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Deterrence is widely defined as any use of threats (implicit or explicit) or limited force intended to dissuade an actor from taking an action (i.e. maintain the status quo). Deterrence is unlike compellence, which is the attempt to get an actor (such as a state) to take an action (i.e. alter the status quo). Both are forms of coercion. Compellence has been characterized as harder to successfully implement than deterrence. Deterrence also tends to be distinguished from defense or the use of full force in wartime.
Deterrence is most likely to be successful when a prospective attacker believes that the probability of success is low and the costs of attack are high. The central problem of deterrence is to credibly communicate threats. Deterrence does not necessarily require military superiority.
General deterrence is considered successful when an actor who would otherwise take an action refrains from doing so due to the consequences that the deterrer is perceived likely to take. Immediate deterrence is considered successful when an actor seriously contemplating military force or taking an action refrains from doing so. Rational deterrence theory holds that an attacker will be deterred if they believe that:
(Probability of deterrer carrying out deterrent threat x Costs if threat carried out) > (Probability of the attacker accomplishing the action x Benefits of the action)
This model is frequently simplified as:
Costs x P(Costs) > Benefits x P(Benefits)