There were 3 wars involving Vietnam in the 20th Century: 1.
Guerilla warfare against the Japanese during WWII (1941-1945), with
US assistance. 2. The French Indochina War (1946-1954); French
Vietnam War. 3. The US Indochina War (1961-1973); US Vietnam
War.
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When Japan surrendered in 1945, ending WWII, the French RETURNED
to Vietnam; they met resistance from the communist backed Viet
Minh, the war was on: 1946-1954.
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It was the result of the 1st Indochina War, aka French Indochina
War (1946-1954).