The Viet Minh or "League for the Independence of Vietnam" were the effective
resistance against Japan's WWII occupation of Vietnam.
A friend of mine who was in the OSS worked with the Viet Minh after his team was dropped into Vietnam in 1945 to obtain reconnaissance data on enemy troop concentrations in preparation for an Allied invasion of
the Japanese held regions.
After the Japanese surrender the Viet Minh gave my friend a letter from Ho Chi Minh requesting help from the west in ending France's control of Vietnam. This was presented at the Paris peace talks, however while the US and president Franklin Roosevelt had an interest in dismantling colonialism, there was no reply or assistance forthcoming. It was my friend's view that the interest of the allies was more focused on the power balance in Europe and perhaps the US was not willing to cross France on the issue of Vietnam.
In any case after being snubbed (I believe there was no official reply to the letter), the Viet Minh sought assistance where it could be had. As with France's resistance effort that proved to be from the communists.
When the Viet Minh finally pushed the French out of Vietnam, the US still held a post-Joseph McCarthy anti-communist sentiment. This virtually guaranteed the US would oppose any regime in southeast Asia which had any communist leanings.
My friend had never revealed his feelings about all this during the US war in Vietnam, however when we talked about his time in OSS many years later, he was clear in his sadness that so much carnage ought to have been avoided. As a realist. However I think he knew that the best intelligence does not guarantee that policy makers will act rationally.