[3] After seven years service in which he reached the rank of captain, he was detailed to the Aviation Section, U.S. Signal Corps in January 1918 during World War I.

In February 1924 he formally transferred to the Air Service to qualify for command of the 88th Observation Squadron at Wilbur Wright Field, Ohio.

On March 2, 1935, Knerr received an assignment promotion (temporary), to colonel when he was made chief of staff of the newly activated General Headquarters Air Force under Maj. Gen. Frank M. Andrews.

In private life, Knerr went to work for the Sperry Corporation Research Laboratories, wrote numerous magazine articles advocating his positions, and conducted a letter-writing campaign against Arnold between 1939 and 1941.

Arnold, now Chief of the Army Air Forces, was initially reluctant, because of the many public and private criticisms Knerr had made of him, but agreed to the return to active duty in August 1941.

However, the Army's Surgeon General, referring to the supposed psychological reasons for his retirement, rejected the reinstatement on medical grounds.

After the United States entered World War II, Knerr was called back to active duty as a colonel in July 1942.

At that time, the Combined Bomber Offensive was being hindered by a low rate of aircraft availability because of maintenance and supply problems, and Knerr was sent to re-organize the system.