Military history of Australia during World War II

Australia entered World War II shortly after the invasion of Poland, declaring war on Germany on 3 September 1939. This was because Australia at this time was a dominion of the British Empire. By the end of the war, almost a million Australians had served in the armed forces, whose military units fought primarily in the European theatre, North African campaign, and the South West Pacific theatre. In addition, Australia came under direct attack for the first time in its post-colonial history. Its casualties from enemy action during the war were 27,073 killed and 23,477 wounded.

In effect, Australia fought two wars between 1939 and 1945 – one against Germany and Italy as part of the British Commonwealth's war effort and the other against Japan in alliance with the United States and Britain. While most Australian forces were withdrawn from the Mediterranean following the outbreak of war in the Pacific, they continued to take part in large numbers in the air offensive against Germany. From 1942 until early 1944, Australian forces played a key role in the Pacific War, making up the majority of Allied strength throughout much of the fighting in the South West Pacific. The military was largely relegated to subsidiary fronts from mid-1944 but continued offensive operations against the Japanese until the war ended.

World War II contributed to major changes in the nation's economy, military and foreign policy. The war accelerated the process of industrialisation, led to the development of a larger peacetime military and began the process with which Australia shifted the focus of its foreign policy from Britain to the United States. The final effects of the war also contributed to the development of a more diverse and cosmopolitan Australian society.

Between World War I and World War II Australia suffered greatly from the Great Depression which started in 1929. This limited Australian defence expenditure and led to a decline in the size and effectiveness of the armed forces during the late 1920s and early 1930s. In the years leading up to the war, Australia followed Britain's policy towards Nazi Germany, supporting first its appeasement of Hitler and then its guarantee of Polish independence.

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