Thailand responded pragmatically to the military and political pressures of World War II. When sporadic fighting broke out between Thai and French forces along Thailand’s eastern frontier in late 1940 and early 1941, 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 kilometers of Laotian territory west of the Mekong and most of the Cambodian province of Battambang to Thailand. The recovery of this lost territory and the regime’s apparent victory over a European colonial power greatly enhanced Phibun’s reputation.
Then, on December 8, 1941, after several hours of fighting between Thai and Japanese troops at Chumphon, Thailand had 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 prearranged with a sympathetic Thai government. Later in the month Phibun signed a mutual defense pact with Japan. Pridi resigned from the cabinet in protest but subsequently accepted the nonpolitical position of regent for the absent Ananda Mahidol.
Under pressure from Japan, the Phibun regime declared war on Britain and the United States in January 1942, but the Thai ambassador in Washington, Seni Pramoj, refused to deliver the declaration to the United States government. Accordingly, the United States refrained from declaring war on Thailand. With American assistance Seni, a conservative aristocrat whose antiJapanese credentials were well established, organized the Free Thai Movement, 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. 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.
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, including portions of the Shan states in Burma and the four northernmost Malay states. Japan meanwhile had stationed 150,000 troops on Thai soil and built the infamous "death railway" through Thailand using Allied prisoners of war.
As the war dragged on, however, the Japanese presence grew more irksome. Trade came to a halt, and Japanese military personnel requisitioning supplies increasingly dealt with Thailand as a conquered territory rather than as an ally. Allied bombing raids 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 the Phibun regime and the military. In June 1944, Phibun was forced from office and replaced by the first predominantly civilian government since the 1932 coup.