Once on active service, he was hit in the leg by shrapnel near Bazentin, France on 9 August 1916, during the Battle of the Somme. The leg was marked for amputation but was saved after being operated on by a German POW surgeon. After recovering back in Britain, Crossan was posted to the 4th Battalion to serve in the Sinai and Palestine Campaign and he was present during the Battle of Jerusalem. He was posted back to the Western Front in 1918 and was gassed in April that year.