The New Look policy, though initially useful, quickly became obsolete with the introduction of inter-continental delivery systems that undermined the credibility of a deterrence threat. The cornerstone of U.S. and European defense strategy was then threatened as the U.S. could no longer rely on nuclear threats to provide security for it and its allies.

John F. Kennedy won the presidency by claiming that the Republican Party had allowed the U.S. to fall behind the Soviets into a missile gap. Upon entering office Kennedy cited General Maxwell Taylor’s book The Uncertain Trumpet to Congress for its conclusion that massive retaliation left the U.S. with only two choices: defeat on the ground or the resort to the use of nuclear weapons.

Technology had improved since massive retaliation was adopted. Improvements in communication and transportation meant U.S. forces could be deployed more effectively, quickly, and flexibly than before. Advisers persuaded Kennedy that having multiple options would allow the president to apply the appropriate amount of force at the right place without risking escalation or losing alternatives. This would improve credibility for deterrence as the U.S. would now have low-intensity options and therefore would be more likely to use them, rather than massive retaliation’s all-or-nothing options.

Flexible Response was implemented to develop several options across the spectrum of warfare, other than the nuclear option, for quickly dealing with enemy aggression. In addition, the survivability of the retaliatory capability was stressed, leading to the diversification of the strategic force, development of the strategic triad, and half the Strategic Air Command force being put on permanent alert status.

The Kennedy doctrine did not include the ability to fight nuclear wars because of the idea that it would undermine deterrence, was technologically unworkable, would fuel the arms race, and was not politically feasible.