Raised as an Australian Imperial Force (AIF) battalion in March 1915 as part of the 5th Brigade of the 2nd Division, the 18th Battalion was formed at Liverpool, New South Wales. Following training, the battalion was sent to Egypt, and from there to Gallipoli in August 1915 as part of a wave of reinforcements that arrived after the initial landing. There it participated in the costly August Offensive, during which it was sent to Damakjelik Bair. From there, they played a mainly defensive role in the campaign, apart from playing a minor role in the attack on Hill 60, Beginning the battle with about 1,000 men, by the end the battalion could muster only 386. Following that, the battalion was placed in reserve behind the 5th Brigade's positions between Walker's Ridge and Quinn's Post, until they were withdrawn in December 1915. Their main area of responsibility during this time was Courtney's Post.