Nuclear deterrence is the threat of nuclear retaliation for a proscribed behavior, generally an attack upon the threatening state. The theory of nuclear deterrence posits that such threat, if perceived as real and likely to cause sufficient devastation, will prevent an attack or other proscribed behavior from occurring.
Each country that has developed nuclear weapons has justified doing so by the pursuit of nuclear deterrence. The security of not only the nuclear weapon states but of civilization has rested upon the reliability of the theory of nuclear deterrence. Vast numbers of people throughout the world believe the myth that nuclear deterrence contributes to the security of the planet and perhaps to their personal security and that of their family. But does it? Many experts think not.
The Deterrence Myth
This essay by David Barash from January 2018 makes the case that deterrence is not remotely as compelling a principle as its reputation suggests.
The Nuclear Age Peace Foundation has produced a short animated video outlining the shortfalls and dangers of nuclear deterrence.