This monograph is essentially a treatment of the manpower aspects of military mobilization. Its primary object is to provide a more comprehensive record of military mobilizations in the United States for the use of General Staff officers and students in the Army school system than has been available before in a single work. Since it is undoubtedly true that mobilization errors have been repeated because the lessons of previous mobilizations have not been readily available, it is hoped that this study will assist the mobilization planners of the future in eliminating such errors. The material will also assist the thoughtful civilian in understanding some of the basic problems of national security. The study ends with the mobilization for World War II. Because of the swift flow of events since 1945, it is merely background for the rearmament of the United States culminating in the Korean operations, and an additional monograph will be necessary to record the mobilization developments and lessons up to the present time.