Basically a Mexican standoff between prominent countries that possess nuclear weapons and have different polictical agendas. Causes the entire world to be gripped in a sort of doomsday fatalism. Also results in peace talks to boil down to a pissing match in which each side takes a "I'll-throw-down-my-gun-if-you-throw-down-your-gun-first" attitude.
In "Beneath the Planet of the Apes" the apes may have survived if they had thier own nukes. The mutant humans wouldn't have used thier nuke because of fear of mutually assureddestruction. Which didn't matter anyway, because they all ended up dead, like a bunch of pork rinds.
Mutually Assured Destruction is where two parties end up destroying each other in the end. This can happen at the end of narcissisticrelationships
David and I's relationship devolved to a platform of mutually assured destruction. He nearly took my life, so I made sure to destroy his reputation. He took my money, so I took his. He cheated on me, so I cheated on him. We were both nearly destroyed by the end.
The recognition that the use of nuclear weapons between any two nuclear powers (or nuclear-allied powers) will result in mutual destruction. The reason nuclear weapons are so useful (and can never be used).