In the old days, Ayutthaya was a large city and a feudal hegemony that other weaker towns/cities must give tribute to, but Ayutthaya did not directly rule subordinated cities far away, only the near ones whose rulers were sent/appointed from Ayutthaya King.

Siam is the modern state in the truist sense, starting from the reign of King Rama V when he abolished feudal system and sent mandarins to govern all the cities within the kingdom.

Thailand name is adopted by the strongman Field Mashall Paek Pibulsongkram to foster nationalism during Rama VIII reign. Pibulsongkram was only a minor rank Army officer when the military/civilians staged the coup to topple absolute monarchy in 1932. He rose quickly and commanded absolute power after King Rama VII, who was toppled from power, died and the young boy King Rama VIII ascended the throne.