In foreign affairs, deterrence is a strategy intended to dissuade an adversary from undertaking an action not yet started, or to prevent them from doing something that another state desires. 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. A credible nuclear deterrent, Bernard Brodie wrote in 1959, must be always at the ready, yet never used.