Thonburi and Bangkok period

After more than 400 years of power, in 1767, the Kingdom of Ayutthaya was brought down by invading Burmese armies, its capital burned, and the territory split. General Taksin managed to reunite the Thai kingdom from his new capital Thonburi and declared himself king in 1769. However, Taksin allegedly became mad, and he was deposed, taken prisoner, and executed in 1782. General Chakri succeeded him in 1782 as Rama I, the first king of the Chakri dynasty. In the same year, he founded the new capital city Bangkok, across the Chao Phraya River from Thonburi, Taksin's capital. In the 1790s Burma was defeated and
driven out of Siam, as it was then called. Lanna also became free of Burmese occupation, but the king of a new dynasty who was installed in the 1790s was effectively a tributary ruler of the Chakri monarch.
The heirs of Rama I became increasingly concerned with the threat of European colonialism after British victories in neighboring Burma in 1826. The first Thai recognition of Western power in the region was the Treaty of Amity and Commerce with the United Kingdom in 1826. In 1833, the United States began diplomatic exchanges with Siam, as Thailand was called until 1939, and again between 1945 and 1949. However, it was during the later reigns of King Mongkut (1804–1868), and his son King Chulalongkorn (1853–1910), that Thailand established firm rapprochement with Western powers. It is a widely held view in Thailand that the diplomatic skills of these monarchs, combined with the modernizing reforms of the Thai Government, made Siam the only country in Southeast Asia to avoid European.
colonization. This is reflected in the country's modern name, Prathet Thai or Thailand, used since 1939 (although the name was reverted to Siam during 1945–49), in which prathet means "nation".
The Anglo-Siamese Treaty of 1909 defined the modern border between Siam and British Malaya by securing Thai authority over the provinces of Pattani, Yala, Narathiwat, and Satun, which were previously part of the semi‐independent Malay sultanates of Pattani and Kedah. A series of treaties with France fixed the country's current eastern border with Laos and Cambodia.