The Portuguese were the first Europeans to give an account of Siam. After their conquest of Malacca in 1511, they sent a diplomat, Duarte Fernandes, to Ayutthaya.
A century later, on August 15 1612, an East India Company merchantman, The Globe, arrived bearing a letter from James I.
Ayutthaya may have been the largest city in the world in the seventeenth century, with a population of a million. Trade flourished, especially with the Dutch, French, Chinese and Japanese.
Episode of Constantine Phaulkon. Siam became more closed after the 1688 revolution (last post but one).
The heirs of Rama I became concerned with the threat of European colonialism after British victories in Burma in 1826.
They signed a series of treaties with foreign powers: the Burney Treaty with Britain in 1826, treaties with the US in 1833 and 1856. Others with other powers during the reigns of Mongkut or Rama IV (reigned 1851-68) and his son Chulalongkorn or Rama V (reigned 1868-1910).
They were caught between the British in Burma and the French in Laos, Vietnam and Cambodia.
The French had designs on Siam. Britain was Siam’s ally and wanted to preserve it as a buffer state, and so it remained. The Thais gave Britain economic privileges in return.
Their survival was due to a balance of power between Britain and France, but they believe that they also owe it to the diplomatic skills and modernising reforms of Mongkut and Chulalongkorn (1851-1910).
They were nevertheless forced to concede territory, influence or claims (it is often hard to tell them apart), especially to the French. Ie:
Cochinchina, or the extreme south of Vietnam, to France in 1862;
Cambodia to France in 1867; Cambodia had been a pawn in power struggles between Siam and Vietnam since the seventeenth century;
Thai-speaking Shan States in the north to British Burma; various dates (1893 Hansard discussion here);
Laos to France in Franco-Siamese War of 1893; large parts of Laos had come under Siamese control when the unified Lan Xang Laotian kingdom had disintegrated at the beginning of the eighteenth century; there had already been cessions in 1888; more was handed over in 1904 and 1907;
Territory in the south to Britain; but the Anglo-Siamese Treaty of 1909 defined the border between Siam and British Malaya by recognising Thai authority over the semi-Malay areas of Patani, Yala, Narathiwat and Satun.
Siam became the only country in Southeast Asia to avoid European colonisation.
From 1892 to 1924, the Siamese government retained lawyers who specialised in international law. Gustave Rolin-Jaequemyns served as Adviser-General from 1892 to 1902. Edward Strobel, a Harvard law professor, served as American Adviser-General from 1906 until his death in 1908. He was followed by further Harvard professors: Jens Westengard, Francis Sayre and Eldon James.
A coup planned by young military officers was discovered and thwarted in 1912. Compare the Russian revolution of 1905, Persian revolution of 1905-07, the Young Turks of 1908.
Siam declared war against the Central Powers on July 22 1917, mainly to gain favour from Britain and France. In 1918 it sent 1,284 volunteers to the Western Front. The force included 95 qualified pilots and a medical unit. In addition to the Chinese Labour Corps and 140,000 Vietnamese troops and workers drafted by the French, the Siamese troops were the only Southeast Asians to participate in the European theatre.
Siam was given a seat at Versailles and used the opportunity to argue for amendments of nineteenth-century treaties. The US obliged in 1920, France and Britain in 1925, but I am not aware of any territorial concessions. Siam was a founding member of the League of Nations in 1920.
Modern Thai politics begin in 1932, when the military staged its first successful coup and transformed the government of Siam from an absolute to a constitutional monarchy, with a cabinet presided over by a prime minister.