When United States President Woodrow Wilson declared war on Germany in April 1917, it was clear the American entry would eventually turn the tide against the Central Powers (Germany, Austria-Hungary, Ottoman Empire and Bulgaria).
Watching and waiting on the sidelines, King Vajiravudh (Rama VI) considered his options. Although Thailand had remained neutral since the outbreak of the First World War in August 1914 and his nation enjoyed friendly relations with Germany, the King recognised the political value of throwing in his lot with the Allied Powers.
The monarch was convinced that participation would be an "…an excellent opportunity for us to gain equality with other nations." Thailand had suffered from the imperial designs of both the British and French, particularly the latter, losing control of Laos and Cambodia and ceding four southern provinces in the years between 1889 and 1909.
Additionally, Thailand was forced to accept the imposition of extraterritorial rights for the citizens of nations such as France, Britain and the United States and King Rama VI was hopeful that Thai participation in the war would allow a revision of these unequal treaties.
Therefore, on 22 July 1917, despite the misgivings of some members of the Royal government, King Vajiravudh (Rama VI) declared war on Germany and Austria-Hungary. The Thais immediately interned and later seized as war reparations no less than 12 ocean-going ships of the North German Line (NGL).
Thailand sent a small expeditionary force consisting of 1,284 volunteers under the command of Major General Phya Pijaijarnrit (later promoted to Lieutenant General and known as Phya Devahastin) to serve with the British and French forces on the Western Front. Included was a contingent of the Army Air Corps.
The Thais arrived in 1918 and the air personnel began training at the French Army Flying Schools at Avord and Istres. Over 95 men qualified as pilots and some were sent to Bomber School at Le Crotoy, Reconnaissance School at Chapelle-la-Reine, Gunnery School at Biscarosse, and to Fighter Conversion Courses at Piox. According to some sources, Thai pilots made their first sorties in the final weeks of the war, although others claim the Thais finished their training too late to take part.
There was also a medical unit which included nurses and it is claimed these were the only women to serve in the trenches of the Western Front.
The Thai contingent marched in a victory parade in Paris on 19 July 1919 and arrived back in Thailand on 21 September 1919. A war memorial was erected in honour of the troops and stands in Sanam Luang park in Bangkok. Inscribed are the names of the 19 soldiers killed in action on the Western Front.
Thailand also participated in the Versailles Peace Conference (with Articles 135, 136 and 137 devoted to her in the final Treaty of Versailles). In January 1920 Thailand became a founding member of the League of Nations.
On 1 September 1920, King Vajiravudh’s decision to go to war was vindicated when the United States ceded her extraterritorial rights. France, after five years of extensive negotiations relinquished her rights in February 1925 while Britain signed a treaty to the same effect in July the same year.