32 U.S.C. § 102

In accordance with the traditional military policy of the United States, it is essential that the strength and organization of the Army National Guard and the Air National Guard as an integral part of the first line defenses of the United States be maintained and assured at all times. 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 such units of other reserve components as are necessary for a balanced force, shall be ordered to active Federal duty and retained as long as so needed.