 The First Australian Imperial Force ( 1st AIF ) was the main expeditionary force of the Australian Army during World War I. It was formed on 15 August 1914 , following Britain 's declaration of war on Germany , initially with a strength of one infantry division and one light horse brigade . The infantry division subsequently fought at Gallipoli between April and December 1915 and the light horse brigade fought in the Middle East . The remaining forces of the 1st AIF were deployed to France , where the division fought as part of the British Expeditionary Force , and the light horse brigade fought as part of the Australian and New Zealand Army Corps