Thailand in World War II Thailand was invaded by Japan during World War II in the 1941 invasion . At the start of the Pacific War , the Japanese Empire leaned on the Thais to allow passage of Japanese troops on their way to invade British - held Malaya and Burma . This was not popular with much of the population , but the Thai government under Plaek Phibunsongkhram ( known simply as Phibun ) thought it preferable to outright Japanese conquest . After Thailand agreed to let Japanese troops pass through the country , the Japanese then allied with Thailand and later Thailand declared war on the Allies . Thailand was still in control of its armed forces and internal affairs , however , the Japanese desired a bilateral relationship substantially similar to that existing with the puppet state of Manchukuo , which meant that bilateral relations between the two countries were rarely mutually beneficial . Ultimately , the primary aim was to ensure the resolution of Thailand ' s sovereignty and establish an independent nation that would be regarded as an equal by the Allies . A well - organised resistance movement numbered around 90 , 000 Thai guerrillas , supported by many government officials allied to the regent Pridi Phanomyong , was active from 1942 on to fight the Japanese . The partisans provided invaluable espionage services for the Allies , as well as performing some sabotage , and in 1944 helped engineer Phibun ' s downfall . After the war , however , Thailand still received little punishment for its wartime role under Phibun . Franco – Thai War . At the start of World War II , Thailand was ruled by an authoritarian government dominated by a group of civil servants and military officers . Its prime minister was Plaek Phibunsongkhram , an army officer who shared many of his countrymen ' s admiration of fascism and the rapid pace of national development it seemed to afford . Consequently , Phibun cultivated and intensified militarism and nationalism while simultaneously building up a cult of personality using modern propaganda techniques . The regime also revived irredentist claims , stirring up anti - French sentiment and supporting restoration of former Thai territories in Cambodia and Laos . Seeking support against France , Phibun cultivated closer relations with Japan . Faced with American opposition and British hesitancy , Thailand looked to Japan for help in the confrontation with French Indochina . Although the Thai were united in their demand for the return of the lost provinces , Phibun ' s enthusiasm for the Japanese was markedly greater than that of Pridi Phanomyong , and many old conservatives as well viewed the course of the prime minister ' s foreign policy with misgivings . Sporadic fighting between Thai and French forces broke out along Thailand ' s eastern frontier in October 1940 , and culminated in an invasion of Laos and Cambodia in January 1941 . Although French forces suffered badly on land , their navy managed to inflict a crushing defeat on the Thai main fleet at Koh Chang , prompting the Japanese to intervene to mediate the conflict . Japan used its influence with the Vichy regime in France to obtain concessions for Thailand . As a result , France agreed in March 1941 to cede 54 , 000 square kilometres of Laotian territory west of the Mekong and most of the Cambodian province of Battambang to Thailand , which changed its name to Phra Tabong Province . The recovery of this lost territory and the regime ' s apparent victory over a European colonial power greatly enhanced Phibun ' s reputation . But because Japan wanted to maintain both her working relationship with Vichy and the status quo , the Thais were forced to accept only a quarter of the territory that they had demanded , in addition to having to pay six million piastres as a concession to the French . Relations between Japan and Thailand subsequently cooled as a disappointed Phibun switched to courting the British and Americans in the hopes of warding off what he saw as an imminent Japanese invasion . Japanese invasion . On December 8 , 1941 , after several hours of fighting between Thai and Japanese troops , Thailand chose to accede to Japanese demands for access through the country for Japanese forces invading Burma and Malaya . Phibun assured the country that the Japanese action was pre - arranged with a sympathetic Thai government . Occupation and alliance . On December 21 , 1941 , a mutual offensive - defensive alliance pact between the two countries was signed The agreement , revised on December 30 , gave the Japanese full access to Thai railways , roads , airfields , naval bases , warehouses , communications systems and barracks . To facilitate greater economic cooperation , Pridi was removed from the cabinet and offered a seat on the politically impotent Regency Council for the absent king , which he subsequently accepted . Later on , as a result of Japanese pressure , the Thai government declared war on Britain and the United States on January 25 , 1942 . The Japanese also invited Thailand to signed the treaty alongside with the Axis Power unofficially with Manchukuo and Wang Jingwei ' s Reorganized National Government of China . This the Thais , who loathed the idea of being treated on the same level as the two puppet regimes , initially resisted , but ultimately the Japanese had their way . Thai resentment on this issue lasted throughout the war , however , and resulted in Phibun refusing to attend the following year ' s Greater East Asia Conference . Thailand was rewarded for Phibun ' s close cooperation with Japan during the early years of war with the return of further territory that had once been under Bangkok ' s control , namely the four northernmost Malay states . In addition , the Thai Phayap Army was permitted to invade and annex the north - eastern Shan States of Burma . Japan meanwhile had stationed 150 , 000 troops on Thai soil and built the infamous Death Railway through Thailand using Asian labourers and Allied prisoners of war . Although the majority of Thais were initially " intoxicated " with Japan ' s string of brilliant victories in early 1942 , by the end of the year there was widespread resentment as a result of arrogant Japanese behaviour and war - induced economic problems . Even during the early stages of the war there was friction over issues such as the confiscation of Allied property and economic and monetary matters , as well as the treatment of Thailand ' s ethnic Chinese community . A vicious contest for saw mills and teak logs owned by British companies erupted early on , followed by similar disputes over the control of enemy energy and shipping facilities within the country . Other problems were more severe . For a time Germany continued actively purchasing local products , but once shipping difficulties became unsolvable , Japan became Thailand ' s sole significant trading partner . Similarly , Thailand had to rely on the Japanese for consumer goods previously imported from Europe and the United States , which Japan was increasingly unable to provide as the war wore on . A shortage of commodities quickly developed , with inflation soaring and standards of living dropping . Worse still , the Japanese had aggressively claimed the right to import goods duty - free , significantly reducing the Thai government ' s revenues . Things came to a head in December 1942 when an armed confrontation between Japanese troops and Thai villagers and police escalated into a shoot - out in Ratchaburi . Although the Ban Pong incident was promptly and peacefully resolved , it served as " a warning signal that alerted Tokyo to the seriousness of the problems in Thailand " . This led to General Aketo Nakamura being sent to command the newly formed Thailand Garrison Army . Nakamura ' s ability to understand the Thai perspective , combined with his affable personality , significantly helped to improve Thai - Japanese relations . This more conciliatory stance occurred at a moment when the tide began to turn against Japan , something which many within the Thai government recognised . Realising that the Allies had seized the initiative in the war , Phibun , well aware of the troublesome personal predicament his relationship with Tokyo had put him in , started distancing himself from the Japanese . In January 1943 he had two of the Phayap Army ' s divisional commanders arrange the return of a group of Chinese prisoners - of - war as a gesture of friendship designed to open secret negotiations with Chungking . But the prime minister ' s star was waning at a much faster rate than he had thought . With the Allies intensifying their bombing raids on Bangkok , public confidence in Phibun , already tested by his idiosyncratic domestic policies , was sagging fast . His frequent absence from Bangkok led morale to plummet , while a sudden proclamation that the capital and its inhabitants be immediately moved north to malaria - infested Phetchabun was greeted with near - universal bemusement and discontent . And it wasn ' t only the public . The kingdom ' s ruling elite too was becoming increasingly weary of Phibun , whose intimidation and demotion of dissenters within the government served to further unite his opponents , who were gathering around Pridi . Even the Japanese were becoming disaffected with Phibun . That a military scheme lay behind Phibun ' s attempt to relocate the seat of government certainly wasn ' t lost on the Japanese . Remote , with the nearest rail connection being at Phitsanulok , a half - day ' s drive away , Phetchabun ' s main asset was its suitability as a mountainous fortress ; moreover , the site was located in a region where the majority of the Thai army was based . Coinciding with the beginnings of Phibun ' s efforts to distance himself – through prolonged trips to the provinces – from the Japanese was the downfall of Benito Mussolini in Italy , an event which sent shock waves throughout the Thai elite , to the point that an emergency cabinet meeting was convened to discuss the European war situation . Analogies with Italy were soon being made by many : while " Badoglio " became an increasingly popular Thai political epithet , the Japanese envoy in Berlin was advised by Reichsmarschall Göring to keep a close watch on the Thai , lest they turn into an " Oriental Italy . " However , in spite of increasing domestic discontent and Japanese distrust , it would not be until the following year that Phibun ' s political demise would come . Resistance . While the Thai ambassador in London delivered Thailand ' s declaration of war to the British government , the Thai ambassador in Washington DC , Seni Pramoj , refused to do so . Accordingly , the United States refrained from declaring war on Thailand . With American assistance , Seni , a conservative aristocrat whose anti - Japanese credentials were well established , organised the Free Thai Movement , recruiting Thai students in the United States to work with the United States Office of Strategic Services ( OSS ) . That Seni was able to achieve this was due to the State Department ' s decision to act as if Seni continued to represent Thailand , which enabled him to draw on Thai assets frozen by the United States . Despite the reciprocal British declaration of war , a parallel resistance movement was formed by the Thai in Britain . They were organised by two leading students , Snoh Tambuyen and Puey Ungphakorn , and were assisted by members of the self - exiled royal family , including Queen Ramphaiphanni , the widow of King Prajadhipok , and her brother , Prince Suphasawatwongsanit Sawatdiwat . From the office of the regent in Thailand , Pridi ran a clandestine movement that by the end of the war had with Allied aid armed more than 50 , 000 Thai to resist the Japanese occupation . In 1944 he managed to engineer the unseating of Phibun , who was replaced by Khuang Aphaiwong , the son of a minor nobleman and a civilian linked politically with conservatives like Seni . Khuang ' s main task was to continue the charade of collaboration whilst shielding the growing underground movement . This he succeeded to a great extent , convincing not only Nakamura but also the notorious Masanobu Tsuji . By the beginning of 1945 , preparations were actively being pursued for a rising against the Japanese occupiers . Plans for an uprising relied on the success of a quick , surprise strike by a special police unit against the Japanese command structure . The residences of leading officers and the Japanese communications facilities were kept under surveillance . The police assault was to be coordinated with a general attack by the partly mechanised Thai 1st Army against Japanese troops in Bangkok . Fortifications , in the guise of air raid shelters , had been dug at key crossroads , and additional troops had been brought into the city in small groups in civilian clothes . The task of Free Thai forces elsewhere would be to thwart Japanese efforts to reinforce their Bangkok garrison by cutting communications lines and seizing airfields . Pridi had to take into consideration that the Japanese were building up their forces in Thailand , which was likely to become a battlefront in the near future . Previously most Japanese soldiers stationed in Thailand had been support troops , but in December 1944 the local command had been upgraded from garrison status to a field army . The Japanese were gathering supplies and constructing fortifications for a last - ditch defensive effort at Nakhon Nayok , about 100 kilometres north - east of Bangkok . Post - war . The atomic bombings and subsequent Japanese surrender precluded the uprising , however . Pridi immediately issued a declaration stating that Phibun ' s 1942 declaration of war was unconstitutional and legally void , thereby dispensing any need for Thailand to surrender . The Thai armed forces initially attempted to disarm the Japanese garrison , but Nakamura refused , arguing that the matter was for the Allies to decide . Khuang in the meanwhile resigned , citing his previous association with the Japanese as a possible obstacle to Thailand ' s rapprochement with the Allies . A caretaker premier was found in the person of Tawee Boonyaket , a Pridi loyalist . In early September the leading elements of Major - General Geoffrey Charles Evans ' s Indian 7th Infantry Division landed , accompanied by Edwina Mountbatten . Later that month Seni returned from Washington to succeed Tawee as prime minister . It was the first time in over a decade that the government was controlled by civilians . But the ensuing factional scramble for power in late 1945 created political divisions in the ranks of the civilian leaders that destroyed their potential for making a common stand against the resurgent political force of the military in the post - war years . Moreover , the post - war accommodations with the Allies weakened the civilian government . As a result of the contributions made to the Allied war efforts by the Free Thai Movement , the United States , which unlike the other Allies had never officially been at war with Thailand , refrained from dealing with Thailand as an enemy country in post - war peace negotiations . Before signing a peace treaty , however , Britain demanded war reparations in the form of rice shipments to Malaya . An Anglo - Thai Peace Treaty was signed on 1 January 1946 . France refused to permit admission of Thailand to the United Nations until Indochinese territories annexed during the war were returned . The Soviet Union insisted on the repeal of anti - communist legislation .