The 'Osutoraria' plan was the supposed, but highly improbable, Japanese scheme for an invasion of Australia (1941/March 1942).

Early in 1942 elements within the Imperial Japanese Navy proposed an invasion of mainland Australia, but the proposal was opposed by the Imperial Japanese Army and the prime minister, General Hideki Tojo, who regarded it as being unfeasible given Australia’s geography and the strength of the Allied defences. The Japanese military therefore adopted a strategy of isolating mainland Australia from the USA by advancing through the South Pacific to create the conditions in which the Japanese forces could interdict the maritime lines of communication essential to the nourishment and enlargement of the US and Australian forces with Australia and it dependencies, and at the same time produce the same effect on the US and New Zealand forces being enlarged in New Zealand. This offensive was abandoned following the Japanese strategic defeats in the Battle of the Coral Sea and the Battle of Midway during May and June 1942 respectively, and all subsequent Japanese operations in the vicinity of Australia were undertaken with the object of slowing the advance of the Allied forces into the areas seized by Japan in the early phases of the 'Centrifugal Offensive'.

The Japanese were believed to have retained their faith in the validity of this concept even after a pair of small but key undertakings, namely the Battle of Milne Bay, in which the Japanese suffered their first defeat in a land battle at the hand of an Australian brigade, and the Kokoda Trail (or Kokoda Track) campaign, in which the Australian forces prevented the Japanese from reaching and taking Port Moresby on the southern coast of New Guinea late in 1942. The towns of Darwin in the Northern Territory and Broome in Western Australia were attacked by air of a number of occasions, and the fact that Sydney’s harbour was also attacked by two midget submarines certainly served to persuade the Australians and Americans that the Japanese were indeed considering invasion. Moreover, what was perceived as the strong possibility of a Japanese invasion also supported the strategic planning of the 'Brisbane Line'.

However, the Japanese had abandoned any concept of an invasion of Japanese as they fully appreciated that the despatch of sizeable forces for operations farther to the south would inevitably and possibly fatally weaken the Japan positions in China and Manchukuo against any Soviet threat. This was primarily the thinking of the Imperial Japanese Army, but the Imperial Japanese Navy concurred as it was wholly unable to spare the million tons of shipping that any invasion would have required.

In Australia, the government, the military and the people were deeply alarmed, after the fall of Singapore in February 1942, about the possibility of a Japanese invasion. Although Japan never actually planned to invade Australia, widespread fear led to an expansion of Australia’s military and war economy, as well as the establishment of closer links with the USA.

Japan’s successes in the early months of the Pacific War led elements of the Imperial Japanese Navy to propose an invasion of Australia, and in December 1941 such elements had proposed the inclusion of an invasion of northern Australia as one of Japan’s second-stage war objectives after the seizure of South-East Asia had been completed. This proposal was advocated most strongly by Captain Sadatoshi Tomioka, the head of the Navy General Staff’s planning section, on the grounds that the USA would in all probability use Australia as a base to launch a counter-offensive in the South-West Pacific. The navy headquarters argued that such an 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 no universal support for this proposal within the navy, however, and Admiral Isoroku Yamamoto, commander of the Combined Fleet, was adamantly opposed to it.

The Imperial Japanese Army opposed the navy’s proposal as being strategically, operationally and logistically impractical. The army was instead focussed on the defence of the perimeter of Japan’s conquests, and believed that an invasion of Australia would over-extend these defence lines. Moreover, the army was not prepared to release the large number of troops it calculated would be required for any such operation from the Kwantung Army in Korea and Manchukuo as it feared the possibility of a Soviet entry the Pacific War and also wished to preserve an option for a Japanese invasion of Siberia.

Tojo also consistently opposed an invasion of Australia, instead favouring a policy of forcing Australia to submit by cutting its lines of communication with the USA. He believed that Japan had already overextended its lines of communication, and lacked the armed strength and logistical capabilities to mount so vast an extension of Japan’s already over-strained and too thinly spread forces. According to Tojo, Japan expected to take and hold the entirety of New Guinea, to maintain Rabaul as a holding base, and to launch air raids into northern Australia. In speeches before the Japanese Diet on 12 January and 16 February 1942, Tojo claimed Japanese policy was to eradicate the British colonies of Hong Kong and in the Malay peninsula as these were bases used against East Asia, and to turn these places into strongholds for the defence of Greater East Asia. Burma and the Philippine islands group would get independence if they co-operated with Japan; the Netherlands East Indies and Australia would be crushed if they resisted, but in exchange for recognition of Japan’s true intentions would receive help in promoting their welfare and development.

The Imperial Japanese army and navy differed greatly in their calculation of the number of troops which would be required to invade Australia, and this constituted the core of the two services' discussion. In December 1941 the navy calculated that a force of three divisions (totalling something in the order of 45,000 to 60,000 men) would be sufficient to secure Australia’s north-eastern and north-western coastal areas, whereas the army calculated that a force of at least 10 divisions (something in the order of 150,000 to 250,000 men) would be needed. The army’s planners estimated that the transport of this force to Australia would require 1.5 to 2 million tons of shipping, which necessitate a significant delay in the planned return of requisitioned merchant shipping. This invasion force would have been larger than the entire force used to conquer South-East Asia. The army also rejected the navy’s proposal of limiting an invasion of Australia to the seizure of lodgements in the north of Australia as being unrealistic in the likelihood of Allied counter-offensives against these lodgements. As a result of it experiences in China, the army believed that any invasion of Australia would have to involve an attempt to conquer the entire Australian continent, something which was manifestly beyond Japan’s capabilities.

The possibility of an invasion of Australia was discussed by the army and navy on several occasions during February 1942. On 6 February the navy ministry proposed a plan in which eastern Australia would be invaded at the same time that other Japanese forces captured Fiji, Samoa and New Caledonia, and this was again rejected by the army. On 14 February, one day before Singapore fell, the army and navy sections of the Imperial General Headquarters again discussed an invasion of Australia, and in the course of this discussion Tomioka argued that it would be possible to take Australia with a 'token force', but this statement was labelled 'so much gibberish' in the Imperial General Headquarters' diary. Lieutenant General Tomoyuki Yamashita said that after his 25^th Army had taken Singapore, he wished to discuss with Tojo a plan for the invasion of Australia, but Tojo rejected plan on the grounds that it would lengthen Japan’s supply lines, which would be precarious and open to attack.

The dispute between the army and the navy was settled late in February with a decision to isolate rather than to invade Australia. The army continued to maintain its view that invading Australia was impractical, but agreed to extend Japan’s strategic perimeter and cut Australia off from the USA by taking Fiji, Samoa and New Caledonia in 'Fs' (ii). The question of whether to invade Australia was discussed by Imperial General Headquarters for the last time on 27 February, when the army stated that it believed that Australia was defended by some 600,000 men. During another meeting held on 4 March the Imperial General Headquarters formally agreed a Fundamental Outline of Recommendations for Future War Leadership which relegated an invasion of Australia as a 'future option' only if and when all other plans proceeded as planned. This plan was presented to the Emperor Hirohito by Tojo, and in effect ended discussion of an invasion of Australia. 'Fs' (ii) was not implemented, however, as a direct consequence of Japan’s defeats in the Battle of the Coral Sea and Battle of Midway, and was cancelled on 11 July 1942.

As the option of invading Australia was rejected in February 1942 and not subsequently reconsidered, the Japanese attacks on Australia during the war were not precursors to invasion as has sometimes been claimed. The major air raid on Darwin on 19 February 1942 and the attack on Broome on 3 March were in fact undertaken to prevent the Allies from using these towns as bases from which to contest the invasion of the Dutch East Indies. The many subsequent air raids on northern Australia during 1942 and 1943 were for the most part small and aimed at the prevention of attacks on Japanese positions to their north by Allied air units based in the region. The attack on Sydney harbour in May 1942 had the goal of diverting Allied forces from Midway island before the Japanese 'Mi' attempt to capture it, and the subsequent Japanese submarine campaigns off the Australian east coast in 1942 and 1943 were attempts to interrupt the supply line between Australia and New Guinea during the New Guinea campaign. Moreover, the Japanese attempt to capture Port Moresby in south-eastern New Guinea by advancing along the Kokoda Track and undertaking the 'Mo' landing operation and by landing at Milne Bay in 'Ri' between July and September 1942 aimed to capture the town to complete Japan’s defensive perimeter in the region. Once secured, Port Moresby was to have become a base from which Japanese aircraft could dominate the Torres Strait and Coral Sea, and not to support an invasion of Australia.

It is worth noting that Matsu Kikan, a joint army/navy Japanese reconnaissance unit of small size, carried out a brief landing on the Australian mainland during January 1944. The undertaking’s mission was to assess intelligence reports that the the Allies had begun to construct major new bases in the Kimberley area of Western Australia on the southern coast of the Timor Sea. After leaving their base at Kupang in western Timor, the 10 Japanese personnel in a sized fishing vessel crewed by West Timorese civilians made brief visits to the uninhabited Ashmore Reef and Browse Island. On 19 January, the Matsu Kikan party entered York Sound on the mainland. While smoke was seen in hills to the east, the Japanese vessel was anchored and camouflaged with tree branches. Landing parties went ashore near the mouth of the Roe river to reconnoitre the surrounding area for about two hours and film it. On the next day, the Matsu Kikan personnel again reconnoitred the area and then headed back to Kupang. Matsu Kikan spotted no signs of recent human activity, and nothing of military significance was learned. An officer involved with the mission reportedly returned to Japan shortly afterward, where he suggested landing 200 Japanese convicts in Australia, to launch a guerrilla campaign, but this suggestion found no favour.