Kammala jumla mai zuwa: An ba Allen izini a 1913 kuma ya shiga rundunar sojojin Australiya (AIF) a ranar 24 ga Yuni, 1915. Allen ya tashi zuwa Misira tare da ƙarfafawa ga 13th Battalion a watan Agusta. A watan Maris na 1916 an daukaka shi zuwa kyaftin kuma aka sanya shi zuwa 45th Battalion. Ya isa Faransa a ranar 8 ga Yuni 1916 kuma ya yi yaƙi a yakin Pozières a watan Agusta. Allen ya kasance a gaban layin a Somme a cikin 1917 kuma ya jagoranci mutanensa ta hanyar yakin Messines a watan Yuni. Jagorancinsa a cikin asarar da aka yi wa Allen ya sami lambar yabo ta musamman da kuma ci gaba zuwa babban a watan Yuli. Ya ci gaba da jagorantar maza a yaƙi, a yakin Dernancourt a watan Afrilu na 1918, sannan a matsayin mukaddashin Laftanar Kanar, wanda ke kula da Bataliya ta 48, a
Yaƙin Monument Wood.