In pursuance of the Japanese plan to isolate Australia from the United States, on 23 January 1942, five thousand Japanese troops of the elite jungle-trained South Seas Detachment under the command of Major General Tomitaro Horii stormed ashore at Rabaul in the Australian Territory of New Guinea and quickly overwhelmed the small Australian garrison. The Battle for Australia had begun. The Japanese then began to develop Rabaul into a major base for further military operations against Australia and the United States in the South-West Pacific area. With the advantage of this foothold on Australian territory, the first Japanese air raid on Port Moresby in the Australian Territory of Papua took place on 3 February 1942.

To appreciate that Australian territory was physically invaded in 1942 by Japanese troops, it is necessary to make the point that Papua was transferred by Britain to Australia in 1906, and was still an Australian Territory in 1942. New Guinea became an Australian League of Nations Mandate after World War I, and was being administered by Australia on behalf of the League of Nations in 1942. Both territories achieved independence from Australia simultaneously in 1975.

John Curtin, Australia's wartime Prime Minister, demanded the return of Australian troops from the
Middle East to defend Australia from a Japanese invasion. Churchill tried to divert them to Burma.

Prime Minister Curtin recalls the 6th and 7th Divisions for the Defence of Australia

The gravity of the situation caused the Australian Government, led by Prime Minister John Curtin, to decide in February 1942 to recall Australia's AIF 6th and 7th Divisions from the Middle East to defend their own country. This decision was forced on Curtin by a realisation that Britain was more concerned to defend India against the Japanese rather than Australia, and that little material assistance in Australia's defence could be expected from Britain. While the troops of the AIF 6th and 7th Divisions were on route to Australia, with most of their fighting equipment following them aboard slow-moving merchant ships, the British Prime Minister, Winston Churchill, tried to divert them to Rangoon for use against the Japanese invaders of Burma.

However, Curtin stood firm, insisting that the battle-toughened AIF troops were vital to the defence of their own country. At this time Australia was being defended by 250,000 hastily recruited militia troops. Although mostly led by capable AIF officers and NCOs, the raw militia troops were poorly armed and inadequately trained to meet battle-toughened Japanese troops on equal terms. Had Curtin not resisted Churchill, it is likely that Australia's two AIF Divisions would have been swallowed up in the Burma disaster. As for Australia, without its AIF 7th Division, Port Moresby would almost certainly have fallen to the Japanese in September 1942. With Port Moresby in Japanese hands, Japanese bombers would have been able to strike deeply into the Australian mainland and Australia would have been exposed to a very substantial risk of Japanese invasion.

The Curtin government was fully aware that Australia would stand virtually alone and defenceless against the power of Japan unless the United States could be persuaded to help Australia. With the threat of Japanese invasion of Australia becoming ever more likely, Curtin worked tirelessly to persuade the United States to help Australia to resist the Japanese advance.

The United States commits itself to the Defence of Australia

The government of the United States was acutely aware of the strategic importance of Australia to both itself and Japan in 1942. The American Army of the Philippines had been cornered by the invading Japanese on the Bataan Peninsula and faced defeat. The American government had no means of supporting this endangered army because Japan had cut America's direct lines of communication with the Philippines by seizing all American islands between Hawaii and the Philippines. The American government shared the Japanese Navy's appreciation that Australia could be used by the United States as a springboard for a counter-offensive against Japan's southern defensive perimeter.

To save Australia, and recover the Philippines, it would be necessary for the United States to establish secure lines of communication and supply between Hawaii and Australia by means of the intervening South Pacific islands. As the Japanese Naval General Staff feared would happen, the United States began to rush American troops in January 1942 to the New Hebrides, New Caledonia, Fiji, Samoa, Canton, and Palmyra islands. Despite these prompt American efforts to keep the lines of communication with Australia open, the Japanese had been able to reach the island of New Guinea first. On 14 February 1942, the United States responded to the Japanese capture of Rabaul in the Australian Territory of New Guinea by committing itself publicly to the defence of Australia.

The Japanese capture of Rabaul initiated the Battle for Australia

The Japanese landing at Rabaul on 23 January 1942 initiated the Battle for Australia and set the stage for a sequence of eight related and vital battles between 23 January 1942 and 5 March 1943. Those battles, namely, Coral Sea, Midway, Kokoda Track, Milne Bay, the Beachheads, Wau, Guadalcanal and Bismark Sea, are comprised within the context of the Battle for Australia. During the battles of Coral Sea, Midway, Kokoda Track and Guadalcanal, the fate of Australia would hang in the balance.