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. 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.