Desire peace? Then be prepared for war. Or so the Romans believed.
It's an easily understood concept in the abstract. But deterrence still remains a mystical quality in the concrete since it is only acquired with difficulty and yet easily forfeited.The tired democracies of the 1930s learned that lesson when they kept acquiescing to Hitler's serial aggressions.
Hitler's Germany foolishly later attacked a far stronger Soviet Union in 1941, given Moscow's lost deterrence after its lackluster performances in Poland and Finland, its pact with the Nazis, and its recent purges of its own officer corps.
Deterrence is omnipresent and also applies well beyond matters of war and peace. The current crime wave of murder and violent assault in our major cities is the wage of loud efforts to defund the police and contextualize crimes as somehow society's rather than the criminal's fault.
As a result, lawbreakers now believe there is a good chance that robbing people or hurting or killing them might result in monetary gain or at least bloody satisfaction. They no longer fear a likely sentence of 30 years in prison. So, they see little risk in hurting people. And innocents suffer.
Applications to the border, international relations, and other crises at the link.