In the early 1960s, U.S. nuclear strategists realized that a national strategy entirely reliant upon the threat of massive nuclearretaliation was not credible, that most challenges would not rise to the level of justifying nuclear response and that the Soviets would seek to take advantage of that fact.
Second, as an alternative to defenses, national leaders adopted a strategy of deterrence, meaning any enemy foolish enough to launch a nuclear attack would suffer devastating retaliation-in-kind.
The threat of massive retaliation in the nuclear context only deterred all-out nuclear war, but not the many other conflicts that plagued the latter half of the 20th Century that did not rise to the level of provoking a nuclear response.