A defense policy in which a country ensures that it has sufficient military power to deter a potential enemy from making an attack. Deterrence is fundamental to U.S. policy, and underlies all the arguments about the need to keep the military strong. The greatest deterrents are considered to be nuclear weapons. Although they have existed since 1945, they have never been used since the end of World War II. The mere possession of nuclear weapons is sufficient to deter an enemy, because, unless a country's entire nuclear arsenal could be wiped out by a first strike, the destruction caused by the inevitable retaliation would be too great a price to pay. The doctrine of deterrence through nuclear weapons is a paradox: weapons of mass destruction have kept the peace.