[2] He attended Norwich University for a year, and then, in 1913, received an appointment to the United States Military Academy (USMA) at West Point, New York.

[2] Following his graduation on April 20, 1917,[1] exactly two weeks after the American entry into World War I, he was commissioned as a second lieutenant in the Cavalry Branch of the United States Army.

[2][1] He served in France and Belgium after the war, and returned to the U.S. in June 1919 to become a student at the Fort Riley, Kansas Cavalry School.

[5] Harmon's subsequent assignments included the 6th Cavalry Regiment at Fort Oglethorpe, Georgia, and four years as Professor of Military Science and Commandant of Cadets at Norwich University.

[8] The United States entered World War II shortly after Harmon's promotion in December 1941, after the Japanese attack on Pearl Harbor.

[8] Elements of the 2nd Armored Division began to arrive in Algeria, French North Africa, in November 1942, as part of Operation Torch.

[9] His on-site reporting and interventions during the Kasserine Pass battles in February 1943 helped stabilize and reorganize the U.S. II Corps, which had been thrown into disorder after the initial German attack.

Noting that Fredendall seemed out-of-touch (and, at one point, intoxicated), he requested and received permission to go to the front and intervene where necessary to shore up Allied defenses.

[11] While Harmon attributed the lion's share of the blame for the reversal to Fredendall, he also began to question Anderson's leadership abilities with respect to a large command.

[12] Major General Harmon had been in Thala on the Algerian border, witnessing the stubborn resistance of the British Nickforce, which held the vital road leading into the Kasserine Pass against the heavy pressure of the German 10th Panzer Division, which was under Rommel's direct command.

[10] Commanding the British Nickforce was Brigadier Cameron Nicholson, an effective combat leader who kept his remaining forces steady under relentless German hammering.

Inexplicably, the 9th was ordered by Anderson to abandon Thala to the enemy and head for the village of Le Kef, 50 miles away, to defend against an expected German attack.

Nicholson pleaded with the American artillery commander, Brigadier General Stafford LeRoy Irwin, to ignore Anderson's order and stay.

"[10] The 9th's artillery stayed, and with its 48 guns raining a whole year's worth of a (peacetime) allotment of shells,[citation needed] stopped the advancing Germans in their tracks.

Harmon led the 1st Armored Division throughout the rest of the Tunisian campaign, which eventually came to an end in mid-May 1943, with the surrender of almost 250,000 German and Italian soldiers, who subsequently became prisoners of war (POWs).