The Second Australian Imperial Force was the volunteer expeditionary force of the Australian Army in the Second World War. It was formed following the declaration of war on Nazi Germany, with an initial strength of one infantry division and related auxiliary components. After considerable expansion of this force, three divisions were sent to the Middle East and North Africa, while the 8th Division was sent to garrison British Malaya and Singapore.

Expeditionary warfare is a military invasion of a foreign territory, especially away from established bases. Expeditionary forces were in part the antecedent of the modern concept of rapid deployment forces. Traditionally, expeditionary forces were essentially self-sustaining with an organic logistics capability and with a full array of supporting arms.