Siam or as we know it today Thailand, was in one agents words "a double edged sword" and provided Brittan with both friend and enemy.

Before Siam was invaded by the Japanese an agreement by Sir Josiah Crosby and Siam's Prime Minister Luang Pibul was that Brittan would not attack Siam, unless it had been invaded by another country, thus preventing any prior military intervention by Brittan till that event happened.

This was to all in SOE and the military in Malaya disastrous if the Japanese were to launch an attack on Malaya and Singapore from French Indo China (Vietnam Laos and Cambodia) via the Kara Isthmus.

A plan to defeat this had been devised (See Matador on this site) but was not acted upon until it was too late. Japan invaded Siam, but allowed its government to carry on, thus ensuring that the general population of Siam saw no change in Siam's administration, so could not object to anything that it decreed.

Its Finance Minister Luang Pridi  in the Siamese Government was pro British in his views, unfortunately not all in the Siamese Government felt the same. On the 25th January 1942 Siam declared war on Britain and the United States but not China.

Luang Pridi held a series of meetings with those that were against the Japanese intervention, it resulted in the  "Seri Thai" (Free Thai) movement a resistance movement that opposed the Japanese. Also at the time an Oxford undergraduate Chamkad Balankura had formed his own resistance group called the "Ku Chat"  (Thai liberation) theses two were amalgamated to form the "XO" group unfortunately the fledgling guerrilla "XO" group was not recognised by SOE or the British government and were effectively sidelined later

There were various attempts to form some sort of resistance force by SOE using native Siamese and Chinese, these proved less that trustworthy.  In fact one attempt using Chinese ended in costing SOE valuable funds, the persons involved demanding more that the job was worth and departing back to where they came.

Although there were the possibilities of SOE involvement in Siam, none were sufficiently organised due to the lack of political will on the British, Chinese and American sides when the final push was to be made. It had too many players that had their own agendas and with the fall of Japan the necessity faded

Part taken from "Malaya 1941, The fall of a fighting empire" Gilchrist. and records in the TNA. HS 7/120