Wikipedia:
At the outbreak of the Great War (widely known as World War I), Vietnam was part of French Indochina. While seeking to maximize the use of Indochina's natural resources and manpower to fight the war, France cracked down all patriotic movements in Vietnam.[1] The country remained a member of the French Empire, and many Vietnamese fought later in World War I.
The French entry into World War I saw the authorities in Vietnam press-gang thousands of "volunteers" for service in Europe, leading to riots throughout Cochinchina.[citation needed]
Almost 100,000 Vietnamese went to Europe to fight and serve on the French battlefront.[2] They were conscripts, and thousands lost their lives at the Somme and Picardy. Additional Vietnamese were in the Middle East. Exposed to new political ideals and returning to a colonial occupation of their own country (by a ruler that many of them had fought and died for), resulted in some sour attitudes. Many of these troops sought out and joined the Vietnamese nationalist movement focused on overthrowing the French.[1]
A total of 92,411 Vietnamese men from French Indo-China were in the service of France and were distributed in the following formations, of which around 30,000 died:

• 4,800 belonging to the 4 or 5 combat battalions
• 24,212 belonging to the 15 transit battalions
• 9,019 colonial medical staff
• 48,981 colonial labourers

Two battalions of Vietnamese combat soldiers served in France: the 7ème[3] and the 21ème Bataillons de Marche Indochinois, RTIC.[4]
The 7th Battalion was formed from the Vietnamese in Tonkin on February 16, 1916 and landed at Marseilles in September. They underwent training at Frejus until April 1917. On 10 April, the 7th was assigned to the 19th Division but on attachment to the 12th Division.

On the outbreak of World War I many of the French officers and non-commissioned officers of the tirailleurs tonkinois and tirailleurs annamite were recalled to France. The Tonkinese riflemen of the 6e Battalion de Tiralleurs Indochine[5] subsequently saw service on the Western Front near Verdun.[6]