[7] After the outbreak of the First World War in August 1914, Crossan enlisted in McCrae's Battalion of the Royal Scots.
[2] Around the same time he was selected for the Scottish League XI with teammates Peter Nellies, James Low and Harry Graham.
[8] 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.
[2] 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.
[2] He returned to Hearts after the war and featured in another six full seasons, receiving two benefit games and bringing his total number of competitive appearances to over 300[3] before being release on a free transfer in 1925, aged 31.