History wise Japan did Attack Siam for 2 days on Dec 8 1941. Then on Dec 27 41 signed treaty so they were allowed to go thur and attack maylay and other country.
I believe there was a ton of guerilla inf resistances. I could add a event card where they could pop up like they did in Philippines had. I do have an event card for US for that action.
James Dunnigan’s book Victory at Sea: World War II in the Pacific has an interesting section on Siam/Thailand’s odd and complicated situation in WWII. As I recall (I don’t have the book in front of me right now), Siam had an authoritarian leader who was was willing to collaborate with Japan, but there were also pro-Allied elements in the government, in the population at large, and in the diplomatic service abroad. If I remember correctly, for example, Siam technically declared war on the US and/or the UK, but the US and the UK ignored the declaration, either because the Siamese ambassabors refused to pass the declaration along to them or because the US and the UK refused to receive it. And the Allies supposedly got a lot of intelligence during the war from the pro-Allied Siamese factions.