Jacka fought in the Gallipoli Campaign that started on 25 April 1915, when his new division landed at Anzac Cove on the 26th in the Dardanelles, fighting against Turkish defenders on a narrow beach and in the steep ravines and hills above. The position was held by New Zealanders and Australians by digging a series of trenches. The trenches held by the 14th Battalion on 19 May 1915 became known as Courtney's Post, which was where Jacka won the Victoria Cross.
On 19 May 1915, the Turks launched an assault against the Anzac Line, capturing a section of the trench at Courtney's Post; one end of which was guarded by Jacka. For several minutes he fired warning shots into the trench wall until reinforcements arrived, after which he attempted to enter the trench with three others; all but Jacka were either wounded or pinned. It was then decided that while a feint attack was made from the same end, Jacka would attack from the rear. The party then proceeded to engage the Turks with rifle fire, throwing in two bombs as Jacka skirted around to attack from the flank. He climbed out onto "no man's land", entering the trench via the parapet. In the resulting conflict, Jacka shot five Turkish soldiers and bayoneted two others, forcing the remainder to flee the trench; he then held the trench alone for the remainder of the night. Jacka's platoon commander, Lieutenant Crabbe, informed him the following morning that he would be recommended for his bravery.