He joined the AIF, the Australian Imperial Force, on 18 September 1914. His battalion was sent to Cairo Egypt for training. They landed at Gallipoli on 26 April, 1915, the second day of the Gallipoli campaign. On 19 May the Turkish army started a large attack to push the ANZACs back into the sea. The Turks were able to capture a small section of a trench at Courtney's Post. Jacka jumped into the trench by himself and killed most of the Turkish soldiers in it. For this he was awarded the Victoria Cross.