That's why the Cold War brought on M.A.D (mutually assured destruction). Both USSR and the US knew that if one started the war the entire world would be destroyed with nukes hitting left right and center. It was literally for the sake of humanity that they didn't kill each other.
Fun/Sad Fact: because of the cold war there was abundance of scientists and intelligence in the world. now that the war was over the nukes are sitting there doing nothing. naturally you would defuse them but the problem was that the people who created them and knew how to defuse them were either too old or dead and couldn't do it. So it's taking a lot of time for countries to defuse them. It's a lot harder then it sounds. This generation is dumber then the last and overall ambition for intelligence has gone down.
video showing every single nuclear launch in history