Harper was ranked 83rd in his class of 139,[2] which included future four-star generals J. Lawton Collins, Matthew Ridgway, and Mark W.

He then served with the Reserve Officers Training Corps (ROTC) in Ames, Iowa, from 12 June 1925 until 1 September 1929, when he returned to troop duty with the 15th Field Artillery at Fort Sam Houston, Texas.

[4] He was promoted to major on 1 November 1932, and attended the United States Army Command and General Staff College at Fort Leavenworth, Kansas, from 1 September 1934 to 10 June 1936.

On 11 December 1941, shortly after the United States entered World War II, he became the commander of the I Corps Artillery at Fort Jackson, South Carolina, with the wartime rank of colonel.

In April 1942, he assumed command of the 30th Infantry Division Artillery at Camp Blanding, Florida, with the temporary rank of brigadier general from 28 May 1942.

[12] For his service in the Mariana Islands, Harper was awarded an oak leaf cluster to his Legion of Merit and the Bronze Star Medal.

[6] Harper then assumed command of the 98th Infantry Division in Hawaii on 22 October 1944, with the temporary rank of major general from 6 February 1945.

He then served under Major General James Van Fleet as deputy commander of the American Mission to Greece during the Greek Civil War.

On 1 December he joined the staff of Commander-in-Chief, United States Naval Forces Eastern Atlantic and Mediterranean, based in London.