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.
Structure US Code
Subtitle E— Reserve Components
PART I— ORGANIZATION AND ADMINISTRATION
CHAPTER 1003— RESERVE COMPONENTS GENERALLY
§ 10101. Reserve components named
§ 10102. Purpose of reserve components
§ 10102a. Deployment prioritization and readiness of Army components
§ 10103. Basic policy for order into Federal service
§ 10104. Army Reserve: composition
§ 10105. Army National Guard of the United States: composition
§ 10106. Army National Guard: when a component of the Army
§ 10107. Army National Guard of the United States: status when not in Federal service
§ 10108. Navy Reserve: administration
§ 10109. Marine Corps Reserve: administration
§ 10110. Air Force Reserve: composition
§ 10111. Air National Guard of the United States: composition
§ 10112. Air National Guard: when a component of the Air Force
§ 10113. Air National Guard of the United States: status when not in Federal service