Free Thai Movement The Free Thai Movement ( ; ) was a Thai underground resistance movement against Imperial Japan during World War II . Seri Thai were an important source of military intelligence for the Allies in the region , and were notable for being the only World War II resistance movement to use fighter aircraft of its own . Background . In the aftermath of the Japanese invasion of Thailand on 7 / 8 December 1941 , the regime of Plaek Pibulsonggram ( Phibun ) declared war on Britain and the United States on 25 January 1942 . Seni Pramoj , the Thai ambassador in Washington , refused to deliver the declaration to the United States government . Accordingly , the United States refrained from declaring war on Thailand . Seni , a conservative aristocrat whose anti - Japanese credentials were well established , organized the Free Thai Movement with American assistance , recruiting Thai students in the United States to work with the United States Office of Strategic Services ( OSS ) . The OSS trained Thai personnel for underground activities , and units were readied to infiltrate Thailand . By the end of the war , more than 50 , 000 Thai had been trained and armed to resist the Japanese . World War II and Japanese occupation . Phibun ' s alliance with Japan during the early years of war was initially popular . The Thai army joined Japan ' s Burma Campaign with the goal of recovering part of the Shan states previously given over to Britain by the Treaty of Yandabo . They gained the return of the four northernmost Malay states lost in the Anglo - Siamese Treaty of 1909 , and with Japanese mediation in the Franco – Thai war they also recovered territory lost in the Franco - Siamese War of 1893 . But Japan stationed 150 , 000 troops on Thai soil . As the war dragged on , the Japanese increasingly dealt with Thailand as a conquered territory rather than as an ally . Though the United States had not officially declared war , on 26 December 1942 , US Tenth Air Force bombers based in India launched the first major bombing raid that damaged Bangkok and other targets and caused several thousand casualties . Public opinion and , even more important , the sympathies of the civilian political elite , moved perceptibly against Phibun ' s alliance with Japan . Pridi and the Civilian Regime , 1944 – 47 . In June 1944 , Phibun was forced out of his office and replaced by the first predominantly civilian government since the 1932 coup . Bombing raids continued , a B - 29 raid on Bangkok destroying two key power plants on 14 April 1945 . ; nevertheless , the Seri Thai network was effective in the rescue of downed allied airmen . The new government was headed by Khuang Aphaiwong , a civilian linked politically with conservatives such as Seni . The most influential figure in the regime , however , was Pridi , whose anti - Japanese views were increasingly attractive to the Thai . In the last year of the war , Allied agents were tacitly given free access by Bangkok . As the war came to an end , Thailand repudiated its wartime agreements with Japan . The civilian leaders , however , were unable to achieve unity . After a falling - out with Pridi , Khuang was replaced as prime minister by the regent ' s nominee , Seni , who had returned to Thailand from his post as leader of the Free Thai movement in Washington . The scramble for power among factions 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 postwar years . Postwar accommodations with the Allies also 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 postwar peace negotiations . Before signing a peace treaty , however , Britain demanded war reparations in the form of rice for shipment to Malaya , and France refused to permit admission of Thailand to the United Nations ( UN ) until Indochinese territories annexed during the war were returned . The Soviet Union insisted on the repeal of anti - communist legislation . Heritage . Sakon Nakhon historical attractions include a cave well camouflage by lush vegetable called Tham Seree Thai ( ถ้ำเสรีไทย ) that the movement used for storage of arms and food during World War II .