On December 8th Japanese forces invaded South East Asia. They bombed the Thai capital Bangkok. Quickly the Thai government signs a heavy handed treaty with the Japanese. The Thai government signed the agreement while most of the Thai people believe their government betrayed them. Under pressure from the Japanese Thailand declares war on the United states and the United Kingdom. Only the British declare war on Thailand.
In the United States the ambassador from Thailand meets with the State Department with the declaration of war in his pocket, Seni Pramoj convinces the US government to give him control of Thai assets in the United States and to contact Thai students in the United States and to train them to work as allies to the United States in Thailand. This is the beginning of the Free Thai Movement, the Seri Thai.
The Seri Thai agents would pass locations of Japanese locations through out Thailand to the American and the Allies would bomb these positions. The Seri Thai would also try to find downed pilots before the Japanese and get them back to the Allies.
The cruelest part of the war in Thailand was in Hellfire Pass along the Mae Klong. This is fictionalized in the movie The Bridge Over the River Kwai. The Japanese used POWs as well as Thai laborers to build a bridge over the river to complete a railroad into Burma. The loss of life in this construction was severe. Most of the dead were Dutch POWs from the Dutch East Indies that the Japanese captured.