During WW2, Fletcher Prowty [0] told a story of surplus arms shipments making their way Vietnam in '45. Why? because of a coming war in Vietnam. The US supported the French in SEA I suspect under the "Truman Doctrine" [1] The French influence stopped in '54 when "Dien Bien Phu" fell. This is the pre-history of the US involvement in Vietnam.

[0] https://en.wikipedia.org/wiki/L._Fletcher_Prouty#World_War_I...

[1] https://history.state.gov/milestones/1945-1952/truman-doctri...

[2] https://history.state.gov/milestones/1953-1960/dien-bien-phu