While flying a Mohawk V with 5 Squadron SAAF, he was shot down over North Africa by a Luftwaffe fighter plane, a Bf 109, on 11 July 1942 and was a prisoner of war [3] until 4 May 1945.

[4] He was sent to Stalag Luft 111 in 1942 where he was reunited with his old friend Roger Bushell (Big X) just before the camp was liberated in 1945 he helped his neighbour in the next bed Paul Brickhill to successfully hide his manuscript of The Great Escape by wrapping it round his forearm and covering it with plaster of paris.

In 1955, he left the South African Permanent Force to join the Army of the Federation of Rhodesia and Nyasaland as a major.

On the dissolution of the Federation, Coster joined the Rhodesian Army, in which he rose to the rank of lieutenant-general and was appointed as general officer commanding.

[7] Coster was awarded four medals for his wartime service and he became an Officer of the Order of the British Empire (OBE) in the 1964 New Year Honours.