Deterrence Theory

Deterrence is successful when nothing happens and it's very hard to measure it (Dr. Adam B Lowther, Air Force Research Institute, Air University)

Deterrence is the action of discouraging an action or event through instilling doubt of fear of the consequences. Successful deterrence - achieve defense goal without fighting.

the basic idea of deterrence is you're trying to prevent someone fof attacking you. you're trying to prevent from an attack or the deter an attack on yourself. whether you as an individual or you as a gorup of people or you as a state country.

most deterrence was achieved by denial that is to say an attempt to deny your possible attacker their objectives. typically this was achieved through things like building up a big army or building lots of castles and fortifications of walls this sort of thing and the whole idea then was to deny your opponent the prospect of an easy victory as a reasonable cost. if by building up like a big army an building lots of castles and fortifications, you had made it very difficult for anyone to believe that they could attack you with any chances of success. the whole idea was they were deterred from attacking you. they wouldn't attack you in the first place. the idea was prevent someone for attacking you by making it very expensive to do so and by denying them the possibility of a good chance of victory.

If you attack me then I will attack you in the way that inflicts so much damage in your country. you couldn't possibly hope to survive and the logic there was you would never then be attacked in the first place an attacker would be deterred because of the threat of punishment.

The Whole idea of nuclear deterrence is it's based on the threat of punishment. you prevent someone from attacking you and you achieved this by presenting any possible attacker with the prospect of incurring unacceptable 

Source : https://www.youtube.com/watch?v=rJjC6nLX4RM