|Siam's Expeditionary Force Parading in Paris, 14 July 1919|
King Vajiravudh of Siam, on 22 July 1917, decided to declare war on the Central Powers and joined the Entente Powers in their fight on the Western Front. He sent a volunteer corps translated in some sources as the "Siamese Expeditionary Force."
Thousands of candidates applied, but only 1,233 men were selected. The Siamese volunteers were initially commanded by General HRH Chakrapong Phuvanart, brother of the King . The force included air, transport, and medical personnel.
|Volunteers Memorial, Bangkok|
After primary training, the force departed Siam on 19 June 1918. Their training in theater was still under way when the war ended, but some of the units became involved in the occupation of the Rhineland after the Armistice. One of their missions was to transport Allied troops across the Rhine river into Mainz.
Although Siam’s participation militarily was minimal, the result was the revision or complete cancellation of unequal treaties with the United States, France and the British Empire. The Force was also given the honour of marching in the victory parade under the Arc de Triomphe in Paris.
Yod Sangrungruang sole survivor of the Royal Thai Expeditionary Force died in October 2003 at the age of 106. He was an aircraft mechanic in France and became headman of his village after his return.
By the way, it was on 20 July 1948, the Siamese constituent assembly voted to change the name of Siam to Thailand, the change to come into effect the following year.
Sources: The Thai Military Blog and the St. Mihiel Trip-Wire