Mutually Assured Destruction
"I know not with what weapons World War III will be fought, but World War IV will be fought with sticks and stones."
Mutually Assured Destruction (MAD) was the doctrine that nuclear weapons, if deployed against another nuclear power, should be deployed en masse with the objective of completely destroying the other country's capacity to retaliate, and vice versa. In other words, if either the USA or the USSR ever used nuclear weapons against the other, it would be assured that neither the USA nor the USSR would exist anymore. Naturally this assumes both countries have a large enough stockpile to accomplish this - and they did. It also assumes that neither side has so much more firepower than the other that it could "win" a nuclear war with one sudden, overwhelming strike.
The goal of a MAD strategy is counter-intuitive: it is actually to prevent a nuclear war. The theory goes that if each side knows that there is no way it can survive a nuclear war, it will get too scared to start one.
Needless to say, this theory made people on all sides of the Cold War very nervous. It may come up in hypothetical World War III scenarios or works set in the late-era (1980s) of the Cold War. With the fall of the Soviet Union, MAD has lost its value as the focus shifts to combating terrorists who do not have access to a large stockpile of nuclear weapons nor territory that can be nuked. Non-atomic MAD still exists in relation to the Korean War: North Korea possesses enough mundane artillery aimed at the South Korean capital of Seoul to reduce it to rubble within hours of the war reigniting, but South Korea has undisputed air superiority  that could destroy any North Korean property without resistance.
One of the theories for why the Cold War ended as peacefully as it did is that belief in doctrine prevented the US and USSR from fighting any war directly against each other, for fear it would escalate to nuclear weapons and destroy both powers. Instead, it became a war of ideologies, economics, and proxy wars, and the US eventually out-converted and out-spent the Soviets, who then collapsed under the weight of their own system.
- North lacks both remotely modern aircraft and effective anti-air weapons