1. If you want to carefully analyze when Thailand became a colony, there is no sovereignty at all. Although Thailand was controlled economically by western countries, mainly Britain, and occupied and enslaved by Japan in World War II, the state power was still tightly controlled by Thai people centered on the royal family.
2. Thailand was called Siam before 1939, but in 1855, under the dual pressure of British diplomacy and military, Thailand was forced to sign an unequal treaty, put forward unequal demands such as collecting tariffs, and said that Britain enjoyed extraterritorial jurisdiction in Thailand, which was then a semi-colony. However, in the London Agreement signed in 1898, many powerful colonial countries were vying for the territory of Thailand, and no one was willing to give it to anyone. It is for this reason that Thailand's territory has not been divided and remains independent as a state.
3. Although Thailand was humiliated in sovereignty from the end of 19 to the beginning of the 20th century, its capitalist economy also developed at this time, and many businesses and industries made some progress. The key point is that the nationalism of feudal monarchy prevailed at this time. Under this opportunity, the Thai people demanded the abolition of unequal treaties and the restoration of national independence, and then came to the beginning of the 20th century. Thailand's feudal monarchy took advantage of the contradictions of various imperialists to slightly ease the previous unequal treaties. It is precisely because of the increasingly fierce struggle between imperialists that no one wants to benefit each other, so Thailand remained independent when the First World War broke out.