Defense, United States Department of, executive department of the federal government charged with coordinating and supervising all agencies and functions of the government relating directly to national security and military affairs. Based in the Pentagon, it is divided into three major subsections?the U.S. Army, the U.S. Navy, and the U.S. Air Force. Among the many Defense Dept. agencies are the Missile Defense Agency (see Strategic Defense Initiative), the Defense Advanced Research Projects Agency, the Defense Intelligence Agency, the National Geospatial-Intelligence Agency, and the National Security Agency. The department also operates several joint service schools, including the National War College.
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