Original: Among the many DOD agencies are the Missile Defense Agency , the Defense Advanced Research Projects Agency ( DARPA ) , the Pentagon Force Protection Agency ( PFPA ) , the Defense Intelligence Agency ( DIA ) , the National Geospatial-Intelligence Agency ( NGA ) , and the National Security Agency ( NSA ) .
Simplified: The Department of Defense is made up of the Department of the Army , the Department of the Navy , the Department of the Air Force , the National Security Agency , and the Defense Intelligence Agency .