This was a Japanese unrealised strategic plan to take New Caledonia island as well as the New Hebrides, Fiji and Samoa island groups on the maritime lines of communication between the USA one the one hand, and Australia and New Zealand on the other (spring/summer 1942).
‘Fs’ was adopted in favour of an alternative scheme to invade and conquer Australia. The operation would have used Major General Tomitaro Horii’s South Seas Detachment bolstered by the 144th Regiment, and Major General Kyotake Kawaguchi’s ‘Kawaguchi’ Detachment bolstered by the 41st Regiment and 124th Regiment.
In February 1942, Admiral Isoroku Yamamoto, commander-in-chief of the Combined Fleet and originator of the ‘Ai’ attack on Pearl Harbor as part of Japanese strategic planning for a war with the USA, proposed an immediate invasion of Australia. The details of this concept have been lost, but it seems that Yamamoto developed his plan as a diversionary operation rather than a realistic military scheme for the seizure of the whole of Australia. Yamamoto desired to draw the USA’s strategic thinking away from the development of plans to launch attacks on the Japanese home islands by compelling the US command to respond to the threat, thousands of miles way to the south, posed by even a limited invasion of Australia. This could thereby serve the same purpose as the alternative ‘Fs’ plan as a strategic diversion, but also pave the way under the right circumstances for a later Japanese expansion into and perhaps right through Australia as an element of the ‘Greater East Asia Co-Prosperity Sphere’.
There was considerable interest in such an operation in the Japanese naval staff as a means of gaining control of the strategic area with all its natural and economic resources, or even of taking direct political and military control as the Japanese had done in Formosa and the Mariana, Palau, Caroline and Marshall island groups of their South Pacific Mandate.
An important first step was ‘Mo’ to invade the Australian mandate of Papua in the south-eastern corner of New Guinea for the purpose of taking Port Moresby, its largest town, as a possible springboard for further attacks on Australian territory. ‘Mo’ was supplemented by ‘Re’ to take the Milne Bay area at the extreme south-eastern top of Papua.
Another alleged proposer and/or supporter of the invasion plan was Captain Sadatoshi Tomioka, chief of the plans division in the First Section (Operations) of the Japanese naval staff under the leadership of Rear Admiral Shigeru Fukudome. Admiral Osami Nagano, the Chief of the Navy General Staff, was also in favour of the Australian invasion plan. Tomioka was in fact the officer primarily responsible for the suggestion of Australia as Japan’s next major objective as, he argued, Australia would otherwise probably become the Allied ‘springboard’ for strategic offensives against Japan. To prevent this, Australia would have to be brought under Japanese control or, failing this, isolated from the USA. This plan was rejected by the Japanese army high command, apparently because the army did not have available the minimum ground forces required for such an operation, estimated at 10 divisions.
Another plan proposed by the Japanese navy at about this time was an invasion of Ceylon, possibly linked with an Japanese army invasion of India from Burma. This plan too was rejected by the staff of the Japanese army.
Within the Japanese navy there was a so-called ‘Australia first’ school of thought, including Vice Admiral Chuichi Nagumo and Captain Mitsuo Fuchida of ‘Ai’ fame, who believed in the overriding importance to the Japanese navy of such an objective to the Japanese empire as part of the political and strategic planning for the southern theatre. Lieutenant General Tomoyuki Yamashita, whose 25th Army had conquered Malaya, agreed with Yamamoto’s thinking and even volunteered to lead the invasion. But the plan was opposed by General Hideki Tojo, the Japanese prime minister, who believed that it was beyond Japan’s capabilities.
The Japanese army, in the form of Lieutenant General Nobumasa Tominaga’s 19th Army in the eastern part of the Netherlands East Indies and western New Guinea, created the North Australian Region Group to consider the tactical and strategic issues relevant to any invasion of Australia. The group estimated the strength of the Allied forces in Australia at the time as 350,000 Australians in 10 divisions reinforced by one or two US divisions then arriving, New Zealand units, and 500 aircraft.
In the face of determined opposition to an invasion of Australia by the Japanese army, the Japanese navy fell back on a less ambitious alternative plan to isolate Australia and so prevent the arrival of US manpower, weapons and other equipment. This objective was to be done by the gradual extension of Japanese control over New Guinea, the Solomon islands group, New Caledonia and the other islands of the New Hebrides group and the Fiji islands group, among others, and thus create an outer defence perimeter in the South Pacific.
Yamamoto’s suggestion in partial support of this concept was the landing of between two and four divisions on the Australian north coast, which was very poorly defended. After landing, the Japanese formations would follow the north/south railway line to Adelaide on the coast of South Australia, thereby dividing Australia into eastern and western sections. With Adelaide in Japanese hands, a second force would land on the south-east coast to strike to the north in the direction of Sydney in New South Wales and to the south in the direction of Melbourne in Victoria.
Kenosuke Sato, a theorist working for the Japanese navy, felt that it would be impossible to move a strategically effective force overland from Darwin to Adelaide, and therefore proposed that any Japanese invasion of Australia should be based on a major force of troop transports and warships which would strike from Japan to deliver their troops on the east coast of Queensland between Townsville and Brisbane, where the population was sparse but the communications good, and also on the north coast of the Northern Territory near Darwin and on the north-west coast of Western Australia. Brisbane would have been the first strategic objective. The Japanese believed that Brisbane could be taken quite quickly with a minimum of cost and resistance, and the way would then be open for the seizure of Sydney by overland and amphibious assault.
Sato felt that the invasion should then drive on Melbourne, but believed that by this time the Australians would have surrendered. Sato was to have become the civil administrator of occupied Australia.
It is worth noting that from the late 1920s the Australian government had started to plan for the possibility of a Japanese invasion. The government suspected that the Japanese migrants in the South Pacific and South-East Asia were part of a government-organised Japanese programme of expansion. The Royal Australian Navy was monitoring the activities of the Japanese in New Guinea and the Netherlands East Indies, and also on Thursday island, off the northern tip of Queensland’s Cape York peninsula. Similarly the government had concerns that Japanese and Chinese fishing boats operating illegally in waters to the north of Australia were associated with Japanese espionage activities.
The professional head of the Royal Australian Navy predicted that, in the event of war, the Japanese armed forces would land somewhere on the Cape York peninsula, and another naval expert pointed out the significance of the Japanese South Pacific Mandate as advanced bases and emphasised the vulnerability of Rabaul, on New Britain island, by pointing out that the distance from Rabaul to Sydney was just 1,850 miles (2975 km), to Darwin 1,735 miles (2790 km), and to Singapore 3,185 miles (5125 km). In comparison, the significance of the Japanese South Pacific Mandate as the Japanese intermediate base to the Australian mandated islands was not generally realised: the distance from Truk in the Caroline islands group to Rabaul is a mere 800 miles (1285 km).
During the later 1930s the Australians became even more concerned about the threat of a Japanese invasion, the prime minister’s department studying the Japanese southward advancement policy and monitoring the arrival of possible agents from Japanese companies operating in New Guinea. It was especially worried when the Zaibatsu Nippon Mining Company sent a geologist to investigate the copper ore deposit in Nakanai district on the north-west coast of New Britain in 1937, and when the Nanyo Boeki Kaisha despatched experts to research the goldfields at Wau in New Guinea during 1939.
Tojo finally came down against any plan for an invasion of Australia and recommended that Emperor Hirohito reject it. In particular, Tojo was more interested in using the available forces in other operations on the mainland of Asia, most especially the ‘Otsu’ and ‘Kantokuen’ plans for a Japanese invasion of the Soviet Far East (Siberia), the invasion of Szechwan for the destruction of the Chinese Nationalist government headed by Generalissimo Chiang Kai-shek, and the completion of Japan’s conquest of China, and ‘Plan 21’ for a Japanese invasion of India along with Indian nationalist forces led by Subhas Chandra Bose. Tojo was also concerned that the Japanese transport and merchant fleets were already extended to their limits, and that the USAAF would still be able to deploy Boeing B-17 Flying Fortress heavy bomber to bases in New South Wales for decisive attacks on any invasion of Queensland.
Thus the ‘Australia first’ plan by the Japanese navy was rejected by the Japanese army even more quickly than the proposed invasion of Ceylon. A force of 10 first-line divisions was the minimum required for the Australian operation, and the Japanese army high command was adamant that it did not have this strength available, although the truth was that the Japanese army high command had no interest in the plans promulgated by the Japanese navy.
At this time the Japanese army knew that Germany was planning the major ‘Blau’ offensives to the Volga river and into the Caucasus, and was confident of a German victory. With this achieved, the Japanese army high command wanted to be ready to strike at the weakened USSR’s eastern territories, and therefore held back large forces to check any Soviet aggression or, when the time was right, launch Japanese aggression across the USSR’s Siberian border.
At the suggestion of the Japanese army high command, therefore, Emperor Hirohito ordered a delay in the invasion plan until Japanese forces had taken Burma and joined forces with Indian nationalist forces. Finally, the Battle of the Coral Sea and Battle of Midway, setting the seal of defeat on the ‘Mo’ and ‘Mi’ undertakings, ensured that the Japanese navy’s plan for an invasion of Australia was finally abandoned in favour of the less ambitious and possibly more realistic ‘Fs’.
The Japanese army and navy concurred that ‘Fs’ was a feasible alternative, as only smaller resources would be required for the operation, which would provide air and naval bases for the interdiction of Allied maritime lines of communication across the South Pacific. The scheme was not favoured by Yamamoto, however, as he favoured the destruction of the US Navy’s Pacific Fleet offered by his ‘Mi’ operation, and was ultimately made impossible by the success of the US counter-offensive in the Solomon islands group, where the Japanese loss of Guadalcanal spelled the end of Japanese expansion in this direction after the Japanese had taken Nauru and Ocean islands, to the west of the Gilbert islands group, in ‘Ry’ and the planned springboards for ‘Fs’, on 25/26 August 1942.
On 6 June the Japanese had despatched the minelayer Hirashima from Karatsu Bay on the island of Kyushu in the Japanese home islands for the Palau islands group together with the destroyer Yunagi and auxiliary minelayer Kahoku Maru escorting the convoy intended to launch 'Fs'. This convoy comprised the 6,493-ton Oigawa Maru, 9,685-ton anti-aircraft transport Arizona Maru, 4,960-ton Sanko Maru, 5,086-ton Myoko Maru, 5,415-ton Brisbane Maru and 6,738-ton anti-aircraft transport Yasukawa Maru.