Both sides developed nuclear arsenals, the Soviets initially with the help of spies who stole American secrets. Eventually, the nuclear arsenals were so big that the US and USSR agreed on a strategy appropriately called MAD, which stood for Mutually Assured Destruction. Thanks, Thought Bubble. And yes, nuclear weapons were and are capable of destroying humanity many times over. Not only once or twice did we get close to nuclear war, during the 1962 Cuban Missile Crisis, and then again in 1983 when we forgot to give the Russians the heads up that we were doing some war games which made it look like we had launched a first strike. Our bad! But even though Mutually Assured Destruction prevented direct conflict, there was plenty of hot war in the Cold War.