Mutually assured destruction (MAD) is the idea that if one of the cold war powers tried to nuke the other, there was a good enough warning system that the defender would be able to launch their nukes as well. Therefore both the US and USSR would be destroyed if either one launched nukes. Because they couldn't attack each other outright they fought through proxy wars and spheres of influence. They both tied to spread their ideology and stop the other power from doing the same. This is why we got involved in places like Berlin, Vietnam, and Korea.
