When Congress shall have declared a national emergency or shall have authorized the use of armed land forces of the United States for any purpose requiring the use of troops in excess of those of the regular army, and the president has ordered into the active military service of the United States, to serve therein for the period of the war or emergency, any or all units and members of the National Guard of this state, all forces so ordered into the active military service of the United States shall from the date thereof stand relieved from duty in the National Guard of this state so long as they shall remain in the active military service of the United States, when so provided by the federal law. Upon being relieved from such duty in the military service of the United States, all such individuals and units shall revert to their National Guard status.