The First Australian Imperial Force (1st AIF) was the main expeditionary force of the Australian Army during the First World War. It was formed as the Australian Imperial Force (AIF) following Britain's declaration of war on Germany on 15 August 1914, with an initial strength of one infantry division and one light horse brigade. The infantry division subsequently fought at Gallipoli between April and December 1915, with a newly raised second division, as well as three light horse brigades, reinforcing the committed units.