With the outbreak of the Second World War, Coward was to serve as a pilot and staff officer. His squadron provided air support during the evacuation of the British Expeditionary Force from Dunkirk, between 27 May and 4 June 1940. On 2 June, he was credited with the probable destruction of a German Messerschmitt Bf 109 fighter aircraft. During the Battle of Britain, on 31 August 1940, his squadron was scrambled from RAF Fowlmere to intercept a group of German Dornier Do 215s. He led the second section in attacking the bombers. The weapons of his Spitfire jammed and, having come under fire, the controls were damaged sending the aircraft into a dive. His left foot was almost severed from his leg. Upon bailing out, he could not stand the pain of the swift fall and deployed his parachute. During the now-slower descent he used the radio lead attached to his helmet to tie a tourniquet around his thigh and stem the bleeding. Upon landing in a field, he met a youth who quickly found a doctor. Within half an hour, he was taken to Addenbrooke's Hospital in Cambridge to have his left leg amputated. He was promoted to flight lieutenant on 3 September 1940.