On 15 August 1914, Poole enlisted in the Australian Naval and Military Expeditionary Force to German New Guinea. Poole returned to Australia with that force and he was discharged on 5 March 1915. Like many other members of the expeditionary force, Poole subsequently enlisted in the Australian Imperial Force on 13 May 1915. Poole was posted to the 20th Battalion, and that unit embarked from Sydney on 26 June 1915, on board "HMAT A35 Berrima". On 14 July 1915, he was promoted to lance corporal. The 20th Battalion landed at Gallipoli on 22 August and the unit took up a position at Russell's Top until evacuated on 20 December. The battalion was taken to Egypt for further training and then moved to France, arriving at Marseilles on 25 March 1916. He fought at the Somme and in July was involved in the heavy fighting at Pozières. On 16 August 1916, Poole was promoted to temporary sergeant, later confirmed on 1 December.