"What the bloody hell is it for?"

No discussion of how theories of war have changed since 1945 would be complete without an admiring mention of the doctrine of nuclear deterrence. The historian E.P.Thompson once identified five different meanings that the expression has, at different times, alluded to. I don't have them to hand, but I think they went largely as follows:

1. The principle, immediately post-war, that only the U.S. had nuclear weapons, and thus everyone else was deterred.
2. The principle of the "nuclear umbrella" over NATO countries (and also over the Warsaw Pact countries) which deterred a conventional attack.
3. Mutual Assured Destruction. That nuclear war would result in retaliation and destruction so total and widespread that an aggressor would be deterred.
   
4. Mutual Assured Destruction, second strike variant. The principle that an attacked country would retain enough capability, through the strategic tripod of missiles, bombers and submarines to survive a surprise attack and exterminate an aggressor.
5. The arms race to maintain Mutual Assured Destruction against the possibility of pre-emptive strike by highly accurate MIRV (multiple independent re-entry vehicle) weapons. Thompson referred to it as "mirror logic" that the deadly weapons in their silos should now be considered naked and vulnerable and requiring yet more resources to be consumed in building systems of destruction to protect them.
   

These were later followed by the Strategic Defense Initiative ("Star Wars"), that would make nuclear weapons "obsolete", and the "nuclear winter" theory by which Mutual Destruction was again Assured. But by then people were a little less impressed by the claim that nuclear weapons would be obsolete, as it generally meant they would have to pay a whole lot more taxes in order to build new nuclear weapons.

All of these contradictory theories were known as "deterrence", even though the Cuban Missile Crisis happened in the middle of it all, when the missiles that were supposed to deter war came uncomfortably close to precipitating one.