Before WWI, the US remained neutral. After WWI, the US had fought with the Allied powers against the Axis powers.
Operation Overlord was D-Day, the Allied invasion of Nazi held Europe. It was possibly the most significant event of WW2 in Europe and the biggest ever combined forces operation ever, the largest landing on enemy held beaches ever, the largest movement of troops in a single day ever and involved the largest parachute invasion ever. It was significant in many ways.
Australia had troops stationed in Malaya and Singapore as part of the British force there. When Japan drove south, Australian troops crossed the border into Thailand as well. And Australia's relationship with the British Empire made it difficult for it not to participate in British wars as it had done in the Sudan and Boer Wars and World War 1. At this distance, it is easy to ask why Australia allowed itself to be dragged into far off foreign wars where there was no direct threat to itself, however that was another era where Australia was part of the British Empire, and followed the leader.
Australia declared war on Germany on Sept 3, 1939
then Italy June 11, 1940
Finland, Hungary, Romania Dec 7 ,1941
Japan, Dec 8 1941
Bulgaria, Jan 6, 1942
and finally Thailand on March 2, 1942
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