The First Australian Imperial Force (1st AIF) was the main expeditionary force of the Australian Army during World War I. It was formed as the Australian Imperial Force (the AIF) 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, being reinforced by a second division which was later raised, as well as three light horse brigades. After being evacuated to Egypt the AIF was expanded to five infantry divisions, which were committed to the fighting in France and Belgium along the Western Front in March 1916. A sixth infantry division was partially raised in 1917 in the United Kingdom, but was broken up and used as reinforcements following heavy casualties on the Western Front. Meanwhile, two mounted divisions remained in the Middle East to fight against Turkish forces in the Sinai and Palestine.

An all volunteer force, by the end of the war the AIF had gained a reputation as being a well-trained and highly effective military force, playing a significant role in the final Allied victory. However, this reputation came at a heavy cost with a casualty rate among the highest of any belligerent for the war. The AIF included the Australian Flying Corps (AFC), which consisted of four combat and four training squadrons that were deployed to the United Kingdom, the Western Front and the Middle East throughout the war. After the war, the AFC evolved into the Royal Australian Air Force; the remainder of the 1st AIF was disbanded between 1919 and 1921. After the war the achievements of the AIF and its soldiers, known colloquially as "Diggers", became central to the national mythology of the "Anzac legend". Generally known at the time as the AIF, it is today referred to as the 1st AIF to distinguish it from the Second Australian Imperial Force raised during World War II.