Complex sentence: Deterrence by punishment is a strategy by which governments threaten an immense retaliation if attacked, such that aggressors are deterred if they do not wish to suffer great damage as a result of an aggressive action.
Split into simpler sentences: Deterrence by punishment is a strategy by which governments threaten an immense retaliation if attacked. Aggressors are deterred if they do not wish to suffer such damage as a result of an aggressive action.