Cooper enlisted in the
Australian Imperial Force (AIF) in June 1915. He was commissioned as a
second lieutenant and assigned to the
15th Battalion, serving in
Gallipoli and Egypt. He was promoted lieutenant in March 1916 and captain in April 1916. After transferring to France in June 1916, Cooper was badly wounded in the right leg at the
Battle of Mouquet Farm on 10 August 1916. An official historian of the 15th Battalion concluded that the injury was due to an instance of
friendly fire. Cooper was moved to England where his condition deteriorated, and his leg was amputated at the 3rd London General Hospital in
Wandsworth a month later. After a period recuperating in England, Cooper was transferred to the
Australian Flying Corps as an
adjutant in October 1917. He was appointed acting adjutant of
No. 6 Squadron in January 1918 and returned to France in September 1918 with
No. 4 Squadron as recording officer. He was appointed
Member of the Order of the British Empire (MBE) in 1919. Cooper served with the
Army of Occupation in Germany from November 1918 to February 1919. He subsequently took an eight-month course in textile industries at the
University of Leeds as part of the AIF's repatriation program, before returning to Australia in 1920. He was officially demobilised from the AIF in March 1921. Cooper resumed his farming at Brackenburgh after the war's end, making adjustments for the loss of his leg. He joined the
Legacy Club in Brisbane and the
Returned Sailors' and Soldiers' Imperial League of Australia (RSL). ==Political career==