During the Invasion of Malaya, the 25th Imperial Japanese Army was actually supported by some 20,000 (2 divisions) of the Royal Siamese Army. The Siamese had coordinated in the occupation of Singapore as well as the transferring of the British POWs to camps in Siam and Burma.
Buddy,
The Siamese didn't really have a choice at the time. If their prime minister, Phibulsongkram, had denied the Imperial Japanese Army's request to cross through Thailand, then Japan would have simply invaded and taken Siam by force and quite frankly we could have done it easily. The Siamese , are diplomatic people, they side with whoever is the dominant power. Remember, buddy, that is the reason why they remained 'independent' and were not colonized by European powers. They sacrifice pawns (cambodia, laos, shan) when they have to, in order to save and protect the queen or king (yes i'm using chess terminology here, lol).
In the end, the Siamese cooperated with Japan out of necessity. They did not have the means to resist us. Had they resisted, their entire country would have been pulverized into submission. Luckily that it was not the case.
Impressive. Japan had really armed up Siam during that time. Good to see this.