Dwight D. Eisenhower assumed the presidency convinced that atomic weapons should be employed as essentially conventional tools for war and diplomacy. Supported by the lessons of the Korean War, a conservative fiscal philosophy, advances in nuclear weapons technology, and an asymmetrical conception of containment, that conviction led Eisenhower to formulate a strategic vision that depended primarily upon nuclear weapons for deterring and fighting both general and limited war. During his first term, however, the President's views on nuclear weapons and, thus, U.S. national security strategy, underwent a significant evolution. Although U.S. national security policy, military force structure, and war plans remained firmly based on nuclear weapons, by 1956 Eisenhower's readiness to fulfill the military logic of his "New Look" strategy had all but disappeared. Focusing on decision-making at the highest level, this dissertation describes and explains the evolution of Eisenhower's nuclear outlook, paying particular attention to the mistaken estimates of Soviet capabilities and intentions, and the hypothesis of American vulnerability, which fueled that evolution.