The rapid Allied defeat in the Pacific caused many Australians to fear that the Japanese would invade the Australian mainland . While elements of the Imperial Japanese Navy proposed this in early 1942 , it was judged to be impossible by the Japanese Imperial General Headquarters , which instead adopted a strategy of isolating Australia from the United States by capturing New Guinea , the Solomon Islands , Fiji , Samoa , and New Caledonia . This fact was not known by the Allies at no time , and the Australian military was greatly expanded to meet the threat of invasion . Large numbers of United States Army and Army Air Forces units arrived in Australia in early 1942 , and the Australian military was placed under the overall command of General Douglas MacArthur in March .