As the Cold War unfolded following World War II, America determined that it faced a hostile and expansionist Soviet Union. The growing threat of Soviet long-range aircraft and long-range missiles posed an unprecedented challenge to defending America against attack. In response, the policies of containment and deterrence became the cornerstones of American strategic doctrine, with a heavy reliance on nuclear weapons and strategic air power to discourage any Soviet or Soviet-supported military aggression. During the Eisenhower administration this policy of striking back decisively against any aggressor was known as massive retaliation and evolved into the policy known as mutual assured destruction or MAD in the Kennedy administration. By the early 1960s, the advent of long-range Air Force ballistic missiles and Navy submarine-launched ballistic missiles joined manned bombers in completing America's strategic deterrent triad.