Let us agree from the very beginning that an exact starting point of the Vietnam war cannot be set. The United States entered that war gradually, following certain steps which spread over a 15-year timeline, from 1950 to 1965. And even before that, one needs to take into account the interest France held prior in the region.
In the second half of the 19th century, the French began to take control of Vietnam. At first, their intervention was meant to be a mere protectorate, but it gradually turned into possession by 1939. Together with Cambodia, Vietnam became known as a French colony belonging to French Indochina, to which Laos was also added later on. Certainly, the French journey to dominion met some resistance from the Vietnamese, but none of their riots were successful.