He joined the New Zealand Expeditionary Force in 1939. During the Second World War he was an officer in the 22nd Battalion, rising to the rank of lieutenant colonel. The history group of the Ministry for Culture and Heritage has an interview with Haddon on its website where he describes the events at the Maleme airfield as part of the Battle of Crete. Donald was awarded the Military Cross in 1942, and the Distinguished Service Order in 1945. In 1947 he was made an Officer of the Legion of Merit by the United States. He was twice mentioned in dispatches, in 1941 and 1942. Donald later wrote about his experiences soldiering in Greece, Crete and Egypt in the book "In Peace and War: A Civilian Soldier's Story", published in 2005.