Australia looked to Britain for protection and support, it was after all, Australia's mother land. When Australian troops were sent to the middle East in 1939 without any consulation, Australia needed them to fight for their own country and the impending advances of Japan.
When Japan attacked Darwin in 1942, Britain was nowhere to be found, they were busy fighting their own war and Australia realised they couldn't rely on them any longer.
Australia turned to the United States for support because of the US's great power and loss of faith in Britain.
The US had their own reasons to support them. They knew that they had to stop Japan from advancing, especially because of what happened at Pearl harbour and Australia promised they would have their back and fight for their country.
The US sent huge numbers in army units to help defend and ward off the Japanese.
When Britain decalred war on 4th of August 1914, Australia was still apart of the British empire and therefore had also declared war.
Australian's were excited to participate and head out to fight for their country due to their lack of knowledge of war in general. They had a lot of pride for their country and their mother land (Britain) so they wanted to help and thought that everything would all be over quickly, rather than years.
After 1940, not many Australian units were sectioned in the Asia Pacific or Australia.
They attempted to get them back from the war in