During World War II, official definitions of the requirements of United States national security were extended beyond the defense of the western hemisphere to include preventing any single power dominating Eurasia. This article challenges the commonly expressed view that this change was due to a belief that a strategy of continental defense would no longer suffice to protect the physical safety of the United States. The focus is on the period between Munich and Pearl Harbor when U.S policy moved away from the principle of non-involvement embodied in the neutrality legislation of 1935-37. The role in this process of the argument that America's own safety was dependent upon the European balance of power, particularly because of the dangers posed by the development of aviation and the possible suborning of Latin America, is critically examined. It is argued that the broader conception of America's security requirements reflected both a consciousness of the unique power of the United States to determine the outcome of the war and an implicit belief that the values and interests for which the nation should be prepared to fight extended beyond physical security.