From 1917: War, Peace, and Revolution, by David Stevenson (OUP Oxford, 2017), Kindle pp. 285-286:
Siam declared war on Germany on 22 July 1917. The 1855 Bowring Treaty had limited its tariffs to 3 per cent and secured extraterritorial jurisdiction for British citizens, soon to be followed by those of other Powers. By 1914 Siam had ceded more than one-third of its territory to French Indochina and to the British Malay states. That its core remained independent owed something to it suiting Britain and France to keep Siam as a buffer, and something to King Rama V (reigned 1868–1910) and his advisors. Rama appointed Prince Dewrawangse as foreign minister, who served for thirty-eight years, and by 1914 was vastly experienced and temperamentally cautious. When Rama VI acceded to the throne in 1910, he kept Dewrawangse on.
Siam was less developed than Greece or Brazil. Its population in 1910 was about 8 million, and Bangkok the one substantial city. Its principal export was rice and most of its foreign trade was managed by the British from Singapore and Hong Kong. Its government was an absolute monarchy, untrammelled by representative institutions, in which members of the royal family held key ministerial portfolios and several hundred foreign advisers worked in royal service. Insofar as public opinion existed, it might have been expected to be hostile to France and Britain; but Europe was distant and Germany and Austria-Hungary could not have aided the country. In fact such considerations were outweighed by the personal outlook of King Rama, who had attended Sandhurst and Oxford and undergone officer training with the Durham Light Infantry. In 1915–16 he made donations to widows and orphans of his former regiment, and he and George V exchanged the titles of ‘General’ in each other’s armies, despite German protests that such behaviour was un-neutral.
Unrestricted submarine warfare and America’s appeal to other neutrals to break off relations with Germany started a similar debate in Siam to those in Brazil and China. The initial response to Wilson was that Siam was very remote from the war, and preferred to see how the situation developed. This holding position was primarily due to Dewrawangse, who worked closely with the British minister in Bangkok, Herbert Dering, who in turn advised London that it was best to apply no pressure but let the situation mature, and this recommendation the Foreign Office heeded. Although it might be advantageous to control the nine German steamers in Bangkok harbour and expel the 300 Germans working for the Siamese government, the country had already cooperated in, for example, deporting Indian seditionists, and the advantages from its belligerency were marginal. Dering also feared the Siamese might seek concessions over the unequal treaties. The situation remained unchanged until Rama returned from a visit to the provinces, during which time Dewrawangse (with reluctant acquiescence from an impatient ruler) sounded out Siam’s overseas emissaries. In a Cabinet meeting on 28 May Rama intervened decisively. Dewrawangse reported that the diplomats were divided: the representatives in France and Russia recommended breaking off relations (as did the French and Russian governments), but the London envoy considered it unnecessary. The king, however, said Siam should join the Allies. Previously the Central Powers had seemed to be winning, but American entry altered the equation and delaying meant Britain would end the war with greater leverage than it had now. Rama hoped the unequal treaties could be revised or even abrogated, although he forbade his ministers from saying so. Instead Dewrawangse, who was uneasy but went along, drafted a note that blamed Germany’s persistence in an illegal method of warfare despite Siam’s protests. The government took over the German vessels before their crews could damage them, rounded up the German nationals, and asked the Allies how Siam could help them. When the communications minister voiced concern about running the railways without German experts, Rama replaced him. The kingdom had an army of 12,000–15,000 men, and initially it was not intended to send troops, but in 1918 a contingent of 1,254 volunteers went to France, where nineteen were killed. Siam attended the peace conference and urged amendments to the unequal treaties and recovery of full sovereignty, which America conceded in 1920 and Britain and France in 1925. In relation to the objectives set for it, Siam’s was the most successful of the 1917 interventions, despite the war being followed by a financial crisis. The story underlines how the new conditions forged opportunities for dissatisfied nations to press claims.