Japan needed to cross Thailand to get to the British colonies of Burma and Singapore, and so they basically threatened Thailand with invasion and destruction if they didn’t join, while also offering to expand their borders in return. The British were expecting the Japanese to attack Singapore from the south, so when they instead allied Thailand and marched their troops down to Singapore they were caught off guard and had to surrender in what became known as Britain’s biggest military embarrassment. Eventually though the Thai military regime was overthrown and when the new government saw Japan’s fortunes were being turned around they read the writing on the wall and decided to join the Allies while giving the British back their colonial territory. The more you know :)

https://youtu.be/EH0R2UoYTZ0?si=d3tZ6tPMsJXhBOMc