10 U.S.C. § 10103 - U.S. Code - Unannotated Title 10. Armed Forces § 10103. Basic policy for order into Federal service

Whenever Congress determines that more units and organizations are needed for the national security than are in the regular components of the ground and air forces, the Army National Guard of the United States and the Air National Guard of the United States, or such parts of them as are needed, together with units of other reserve components necessary for a balanced force, shall be ordered to active duty and retained as long as so needed.