We have bad news. Your teacher was wrong. Mutually assured destruction, according to a growing body of scholarship, is a myth. There is no magic theory that renders nuclear war impossible. If you don’t believe us, consider what happened in the 20-plus years from Defense Secretary Robert S. McNamara’s speech articulating mutually assured destruction to the end of the Cold War. If the theory were correct, then neither country would need to alter its nuclear strategies or upgrade its arsenals, which would already ensure mutual annihilation and therefore mutual deterrence. But that is not what happened. Instead, both sides spent billions on larger and more numerous warheads as well as faster and more accurate missiles. They conducted high-risk drills and exercises to prepare for nuclear war, gamed out how to fight such a war and, according to internal documents, worried desperately that one might come about. Two political scientists, Brendan R. Green and Austin Long, pull at this thread in a new academic paper: If mutually assured destruction made nuclear escalation unnecessary, they ask, why was nuclear escalation such a high priority for the United States and Soviet Union? They reach an answer that a growing number of nuclear scholars are circling around — because mutually assured destruction does not exist. Even if Soviet and American leaders both declared as policy that nuclear war was unthinkable, they behaved as if it was very much foreseeable.