US sent military aid to France in 1940 to help them take over Vietnam.
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they supported the non communist country of south Vietnam
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The US supplied massive military aid to South Vietnam. All in vain, as it turned out.
North Vietnam started the war against South Vietnam; the US went to the aid of the South.
While the United States and France were allies during WW2, it's important to note that France already controlled Vietnam and the surrounding area. Known as French-IndoChina, France already had troops, civil administrators, educators, and government officials in place; it made sense for the US to support it's ally.