To this end, it is the intent of the Congress that whenever Congress shall determine that units and organizations are needed for the national security in excess of those of the Regular components of the Ground Forces and the Air Forces, and those in active service under this title [sections
451 to
471a of this Appendix], the National Guard of the United States, both Ground and Air, or such part thereof as may be necessary, together with such units of the Reserve components as are necessary for a balanced force, shall be ordered to active Federal service and continued therein so long as such necessity exists.