The Australian Imperial Force (AIF) was the name given to two all-volunteer Australian Army forces dispatched to fight overseas during World War I and World War II.
Following Federation in 1901, Australia had a very small regular army and reservists in the Australian Citizens Military Forces could not be deployed overseas. Therefeore the AIFs were formed, in 1914 and 1939 respectively.
The two AIFs are distinguished by referring to the World War I contingent as the 1st AIF, and the World War II contingent as the 2nd AIF. During World War I, the Australian Flying Corps, the precursor of the Royal Australian Air Force, was part of the 1st AIF.
The modern Australian Army was established in 1947 and its soldiers may be, and are, deployed anywhere in the world.