Throughout the Cold War, the doctrine of mutually assured destruction required a sufficiently large force that would allow for a massive retaliation even if a first strike eliminated a large portion of a country’s nuclear arsenal. Additionally, during most of the Cold War, delivery systems were not particularly accurate, which required that nuclear weapons have very large yields to reliably strike a target that might be located miles away from the point of detonation (many hydrogen bombs were in the several megaton range). As the accuracy of delivery systems improved, fewer nuclear warheads were required to maintain a credible deterrence threat, leading to a decline in both countries’ arsenals.