1. The Phibun regime was hardly pro-Japanese. In fact, it was pro-Thai – Phibun was one extremely fickle man who acted according to his perception of national and personal interest (not that there’s any real distinction between the two). If you’d bother to read up on the crucial months of 1941 you’d find the Janus-like Phibun doing his best to court the Allies in an attempt to ward off the impending Japanese invasion while at the same time preparing to lay the groundwork for a move into the Japanese camp (should it become necessary). For example, in early December he made an impassioned argument to win support for cooperation with Japan, while on the following day he declared with the same amount of conviction his rejection of Japanese pan-Asianism.
2. Remember that in those days Fascism was very much in vogue. The leaders of the three independent nations of Asia – China, Japan, and Thailand – admired national socialism and its capability to foster the discipline needed to rapidly modernise and strengthen their countries.
Prayoon may have been half-German and leader of the Yuwachon, but it should also be remembered that the man had very little political clout (he was after all rescued from the political wilderness following the conservatives’ debacle in the early ‘30s because Phibun wanted to use him for liaison with the Germans). His advocacy for cooperation only furthered his political isolation. Phibun’s deputy – and therefore the second most powerful man in the army that formed the basis of his power – was General Mangkorn, who even though he headed the 1940 goodwill mission to Tokyo was viewed with suspicion by the Japanese and whose elevation to the post of defence minister was applauded by the British and American.
Sin of course had always been enthusiastic when it came to dealings with the Japanese ever since the Thai naval procurement mission of 1935. This was a result of the IJN’s willingness to assist his service, which had always played second fiddle to its rival, the army. His enthusiasm however was reduced considerably as the war progressed.
3. Thailand’s first objective was to ride out the storm, not to regain her lost territories. Phibun’s first objective was to ride out the storm and to stay in power. Allying with the Japanese was not done out of any ideological fervour; it was a necessity that spared Thailand destruction. Expanding the kingdom’s borders was simply an added bonus for being on the “winning” side of the war, and was probably seen by the ever opportunistic Phu Nam
as a cheap way to increase his personal prestige.
4. In Phibun’s absence the armed forces’ standing orders were to resist any invaders. The fighting against the Japanese on December 8 was no symbolic act to save face; it was the rational thing to do (especially for a population that’s been constantly exhorted to defend their nation’s sovereignty by the regime’s remarkably effective propaganda machine). No different from what the Dutch, Danes, Belgians and Norwegians did in 1940.
5. From mid December 1941 to March 1942 Thailand considered joining the Axis alliance, mainly because many in the country’s leadership felt that the alliance with Japan and the declaration of war on Britain and the US had not sufficiently boosted Thailand’s standing in Axis circles. Such a move however was discouraged by the Japanese, who feared that this would undermine their right of leadership in the Asian sphere.
6. The Thai-Japanese alliance was no smooth affair. The two countries bickered often over such issues as the management of the local Chinese community, the disposition of Allied property in Thailand, as well as various economic matters. Phibun’s refusal to personally attend the Greater East Asian Conference as well as the Thai government’s half-hearted recognition of the Wang Ching-wei government too did nothing to endear them to the Japanese.
As a matter of fact, clashes between Thai and Japanese troops occurred in December 1942 and again in July 1944.
7. Pridi refused to sign the Thai declaration of war; the absence of his signature allowed the post-war government to rescind it. So one could argue that Thailand was never legally at war with the Allies... but just keep in mind that Phibun's government did actively support Japanese military operations.