National defense may refer to:

• National security, a nation's use of military, economic and political power to maintain survival; see also Defense (military)
• National missile defense, a military strategy to shield a country from missiles
• National Defense Service Medal, a decoration of the U.S. military
• National Defense Reserve Fleet, a reserve of U.S. merchant ships
• National Defense University in Washington, D.C.
• National Defense Medical College in Japan
• National Defense Medical Center in Taiwan
• National Defense College of the Philippines
• National Defense Research Committee in the U.S. in the 1940s
• National Defense Education Act
• Government of National Defense in France in the 1870s

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