Self-preservation and military measures to insure the territory of the United States against violation by foreign powers - the subject of this book - ceased to be of serious concern to the United States Government and nation during the nineteenth century. In World War I, the Americans concentrated on the offensive. In World War II, as the authors of this book remark in their Preface, we passed to the offensive so soon and with such force after the United States became engaged that the military provisions for defense have been obscured from view.