 Lahm reverted to his permanent establishment rank of captain on September 9, 1919, and began studies as a "special student" (because his rank had been reduced below that of field grade) at the General Staff College in Washington D.C., through June 1920.The Air Service was made a statutory part of the Army on June 4, 1920, and Lahm applied for transfer to the new branch.He was promoted to major on July 1 and lieutenant colonel on July 2, then transferred in grade to the Air Service on August 6.