Siam is an exonym, a name that is applied externally and used by foreigners. The ethnic Thai referred to their kingdom as ‘Muang Thai’, meaning the country of the Thai people. Officials adopted ‘Thailand’ in English to replace Siam, and ‘Prathetthai’ in Thai language at the end of the 1930s and again in the 1940s. The change occurred as Thai elites, in interaction with Western imperialism, developed new ideas about their political space. ‘Muang’ simply means a any space, from a city up to a whole kingdom or country in size, that is controlled and governed. Conversely, Prathet has the meaning of ‘nation’. The change in terminology was the culmination of Thai state elites transitioned from viewing their political space as a kingdom, and adoption of a modern national vision of the Thai state, subjugating and eliminating smaller subordinate kingdoms and principalities that once were semi-independent but connected to the central Thai kingdom. This transformed the entire area into a Thai national space.

If this interests you, be sure to check out Thongchai Winichakul’s book Siam Mapped.

Siam is Thailand.

Prime Minister Plaek Phibulsongkram, a military dictator in the pre-war + postwar era, was very nationalist and saw nation-building as an important goal. He saw that:

1. Thai people already often referred to themselves as Thais

2. Many immigrants i.e from China were arriving in Thailand.

PM Plaek thus decided that to build a more cohesive nation, it would be proper to change the name of the country to fit with the main ethnic group that inhabited it.