Thailand
On 21 December 1940, a formal military alliance between Thailand under Field Marshal Plaek Pibulsonggram and Japan was concluded. At noon on 25 January 1942, Thailand declared war on the United States and Great Britain. Some Thais supported the alliance, arguing that it was in the national interest, or that it was better sense to ally oneself with a victorious power. Others formed the Free Thai Movement to resist. The Free Thai Movement was supported by Force 136 and the OSS, and provided valuable intelligence from within Thailand. Eventually, when the war turned against the Japanese, Phibun was forced to resign, and a Free Thai-controlled government was formed. A coup was being planned to disrupt the Japanese occupying forces in 1945, but was forestalled by the ending of the war.
Read more about this topic: Force 136, Operations