Chisholm had been a night fighter pilot with
No. 604 Squadron RAF, flying the
Bristol Beaufighter. During the war, he had been credited with seven night aerial victories, one probable and one damaged in 1941. Following a rest period, he returned to operations briefly in 1943 ending his combat career with nine victories. Chisholm championed radar-equipped night fighter intruder operations over Europe to apply pressure to the German air defence system and reduce losses to Bomber Command. He was appointed to the staff of a new organisation, named
No. 100 Group RAF, created in 1943 for this purpose. As second in command of 100 Group from November 1943, his mission in Germany at the end of the war was to gather any useful information on enemy tactics and technology. In early 1944, Chisholm was awarded the Distinguished Service Order (DSO). The citation for the DSO was published in
The London Gazette and read: ==Later life==