Deterrence theory gained increased prominence as a military strategy during the Cold War with regard to the use of nuclear weapons. It took on a unique connotation during this time as an inferior nuclear force, by virtue of its extreme destructive power, could deter a more powerful adversary, provided that this force could be protected against destruction by a surprise attack. Deterrence is a strategy intended to dissuade an adversary from taking an action not yet started, or to prevent them from doing something that another state desires. A credible nuclear deterrent, Bernard Brodie wrote in 1959, must be always at the ready, yet never used.
In Thomas Schelling’s (1966) classic work on deterrence, the concept that military strategy can no longer be defined as the science of military victory is presented. Instead, it is argued that military strategy was now equally, if not more, the art of coercion, of intimidation and deterrence.[2] Schelling says the capacity to harm another state is now used as a motivating factor for other states to avoid it and influence another state’s behavior. To be coercive or deter another state, violence must be anticipated and avoidable by accommodation. It can therefore be summarized that the use of the power to hurt as bargaining power is the foundation of deterrence theory, and is most successful when it is held in reserve.
From Wikipedia: https://en.wikipedia.org/wiki/Deterrence_theory