During which century did Thailand begin opening to the West?

Thailand began opening to the West primarily during the nineteenth century. This period marked significant changes in Thai interactions with Western nations, particularly under King Rama IV (Mongkut) and King Rama V (Chulalongkorn). They enacted various reforms and diplomatic initiatives to modernize the country and prevent colonization, which was a common fate for many Southeast Asian nations during that time. The opening to the West included the establishment of treaties, modernization of infrastructure, and adaptation of Western education and legal systems. This historical context positions the nineteenth century as a pivotal point in Thai history regarding Western engagement, rather than the twentieth or twenty-first centuries, which might imply a different degree of openness or modernization.