This publication, one in the series on Aerospace Education III, deals with the background of the defense system of the United States. Description of different wars in which this country was involved includes the development of new military organizations and different weapons. One chapter is devoted in its entirity to the organizational structure of the present Department of Defense. The last chapter reviews the missions and capabilities of the U.S. Army, Navy and Marine forces and describes some of the more advanced equipment employed by each of these forces. The book is designed to be used in the Air Force ROTC program. (PS)