In early 1942, elements of the Imperial Japanese Navy (IJN) proposed an invasion of mainland Australia. This proposal was opposed by the Imperial Japanese Army and Prime Minister Hideki Tojo, who regarded it as being unfeasible, given Australia's geography and the strength of the Allied defences. Instead, the Japanese military adopted a strategy of isolating mainland Australia from the United States by advancing through the South Pacific. This offensive was abandoned following the Battle of the Coral Sea and the Battle of Midway in May and June 1942, and all subsequent Japanese operations in the vicinity of Australia were undertaken to slow the advance of Allied forces.
This is all despite key battles, including the Battle of Milne Bay, where the Japanese suffered from the first defeat of a land battle at the hands of an Australian Brigade, and the Kokoda Campaign, where the Australians prevented the Japanese reaching Port Moresby, the capital of the Australian Territory of Papua, in late 1942. The Australian townships of Darwin and Broome were attacked a number of times by air as well and the fact that Sydney Harbor was also attacked by two midget submarines certainly would have given the Australians and Americans the impression that the Japanese were considering invasion as a strong possibility. This also supported the strategic planning of the Brisbane Line.
Former Australian War Memorial principal historian Dr Peter Stanley states that the Japanese "army dismissed the idea as 'gibberish', knowing that troops sent further south would weaken Japan in China and in Manchuria against a Soviet threat. Not only did the Japanese army condemn the plan, but the Navy General Staff also deprecated it, unable to spare the million tonnes of shipping the invasion would have consumed."
After the fall of Singapore in February 1942, the Australian government, the military and the people were deeply alarmed by the possibility of a Japanese invasion. Widespread fear led to an expansion of Australia's military and war economy, as well as closer links with the United States.
Japanese proposals
Debate between the Army and Navy
Japan's success in the early months of the Pacific War led elements of the Imperial Japanese Navy to propose invading Australia. In December 1941 the Navy proposed including an invasion of Northern Australia as one of Japan's "stage two" war objectives after South-East Asia was conquered. This proposal was most strongly pushed by Captain Sadatoshi Tomioka, the head of the Navy General Staff's Planning section, on the grounds that the United States was likely to use Australia as a base to launch a counter-offensive in the South-West Pacific. The Navy headquarters argued that this invasion could be carried out by a small landing force as this area of Australia was lightly defended and isolated from Australia's main population centres. There was not universal support for this proposal within the Navy, however, and Isoroku Yamamoto, the commander of the Combined Fleet, consistently opposed it.
The Japanese Army opposed the Navy's proposal as being impractical. The Army's focus was on defending the perimeter of Japan's conquests, and it believed that invading Australia would over-extend these defence lines. Moreover, the Army was not willing to release the large number of troops it calculated was needed for such an operation from the Kwantung Army in Manchuria as it both feared that the Soviet Union would enter the Pacific War and wanted to preserve an option for Japan to invade Siberia.
Prime Minister Hideki Tojo also consistently opposed invading Australia. Instead, Tojo favoured a policy of forcing Australia to submit by cutting its lines of communication with the US. In his last interview before being executed for war crimes Tojo stated,
In speeches before the Diet of Japan on 12 January and 16 February 1942, Tojo claimed Japanese policy was to "eradicate the British colonies at Hong Kong and in the Malay Peninsula as these were 'evil bases used against East Asia', and turn these places into strongholds for the defence of Greater East Asia. Burma and the Philippines would get independence if they co-operated with Japan; the Netherlands East Indies and Australia would be crushed if they resisted; but if they recognised Japan's true intentions would receive help in promoting their welfare and development."