Part of the story below with the rest at this website:
http://www.cpamedia.com/history/thailand_in_shan_state/
On December 8, 1941, Japanese troops invaded Thailand at nine separate points: by land, from Battambang in Cambodia, by air at Don Muang airfield, and by sea in seven amphibious landings between Hua Hin and Pattani on the gulf coast. Despite fierce fighting at points in the south, organised resistance lasted only a few hours. Field Marshal Phibun Songkhram, then Thai ruler, ordered a cease-fire, his government having agreed that to fight the Japanese would be suicidal. At this point Britain and the United States regarded Thailand as an enemy-occupied country and the innocent victim of Japanese aggression.
Unknown to the allies - and to most of his own cabinet - Phibun had other ideas. Influenced, perhaps, by the sinking of Britain's two capital ships,
Repulse and
Prince of Wales, within sixty hours of the outbreak of war in the Far East, he determined to seek an alliance with Japan. On December 14 he signed a secret agreement with the Japanese committing Thai troops to participate in the invasion of Burma, One week later, on December 21, 1941, Phibun signed a formal treaty of alliance with Japan in front of the Emerald Buddha at Wat Phra Kaeo, considered the most sacred object in all Thailand.
Phibun's reward for entering into this alliance was a secret Japanese guarantee to return to Thailand the Malayan provinces ceded to the British in 1909, as well as - with no comparable historical justification - the "lost territories" of Burma's Shan State. In pursuit of these aims, because he believed the Allies beaten, and because it was an auspicious day, on January 25, 1942, Phibun declared war on Britain and the United States.
As is well known, Phibun's action was opposed by most Thais as well as by Thailand's ambassador to Washington, Seni Pramoj, who simply refused to deliver the declaration of war to the US Secretary of State. Phibun, however, was determined that Thailand should both be on the winning side, and that it should benefit from the spoils of victory. Thailand, he announced, was a nation of great warriors who had to learn to adapt and make sacrifices. Part of this process was to recognise who the country's true friends were. Britain could no longer be counted as such a friend, as it had long deprived Thailand of some of its territory. And so... let loose the dogs of war............................................... ...!