This was the British deception plan, by Admiral the Lord Louis Mountbatten’s South-East Asia Command, associated with the planned ‘Zipper’ assault on western Malaya (April/August 1945).
Related to ‘Conscious’ and ‘Slippery’, the ‘Sceptical’ plan was designed to persuade the Japanese that the British forces intended to strike for Bangkok in Thailand by a combination of an overland advance from south-eastern Burma and airborne landings, and for Singapore via the Malacca Strait after establishing forward operating bases in Java and Sumatra in the Japanese-occupied Dutch East Indies.
This was one of many Allied deception undertakings within the wide-ranging ‘Broadaxe’ plan intended to persuade the Japanese not to withdraw forces from South-East Asia and the Pacific to strengthen their forces in the home islands, and within the home islands to deploy their forces in areas other than Kyushu and Honshu that were to be invaded in ‘Olympic’ and ‘Coronet’ respectively.
Within this concept ‘Broadaxe’ sought to inculcate in the minds of the Japanese high command the idea that the Allies intended not to invade Japan until they had secured additional major base areas such as Formosa, the coast of China, Thailand, French Indo-China, Sumatra, the Kurile islands group, Hokkaido island and regions round the Yellow Sea.
Of all the mass of Allied deception plans, only ‘Sceptical’ (i) was implemented to any significant degree before the end of the war. A steady stream of messages delivered to the Japanese by ‘special means’ (supposed double-agents and the like) strongly suggested the ‘Broadaxe’ concept. ‘Sceptical’ (i) was implemented by 'special means' delivery of supposed order of battle information, including the steady movement of units into position, together with rumours about Mountbatten’s intentions. A party of eight supporters of Subhas Chandra Bose’s anti-British Indian movement was captured after being landed by a Japanese submarine and 'turned' top become the major channel by which disinformation was passed to the Japanese, though this ‘Pawnbroker’ channel of communication was interrupted for a month after the British had retaken Rangoon in May 1945 and Subhas Chandra Bose moved his headquarters to Bangkok. After communication had been re-established, ‘Pawnbroker’ once again became vital to the Allied plan.
In July the disposition of Japanese troops seemed progressively less favourable to Allied intentions in South-East Asia, and the South-East Asia Command demanded urgent implementation of the ‘Slippery’ threat to the Kra isthmus. By that time, the pro-Japanese All India Youth League believed it had acquired high-grade sources in the person of an Indian Air Force officer and an Indian army staff officer, and ‘Pawnbroker’ was able to provide what seemed to be very significant information. Late in the effort ‘Ultra’ was able to confirm the high rating that both Bose and the Japanese placed on the reality of the information passed to them by ‘Pawnbroker’, and also to confirm that the Japanese were fully persuaded that Mountbatten intended to attack Bangkok with the notional British XXVI Airborne Corps and to land on the Kra isthmus.
Though it is hard to fix the extent to which deception played a part in the faulty Japanese appraisal of Mountbatten’s intentions, it is cleat that the Japanese did not expect the initial attacks to come in the area really targeted for ‘Zipper’ but instead believed that Mountbatten would first secure air bases on Phuket island and on northern Sumatra, make a major landing around Penang, some 200 miles (320 km) to the north of the actual ‘Zipper’ target area, in November or December, move south along the Malayan west coast after a month, and only then make a major landing in the ‘Zipper’ target area around Port Swettenham and Port Dickson.