How'd the French get there?
What the hell were the French doing in Vietnam anyway?
Compressing a few hundred years of history into a couple of sentences, like other European powers, the French started trading with Asia during the sixteenth century, establishing small communities* throughout southeast Asia. During this period, the area of Vietnam was actually two different kingdoms, both with strong ties to China. China had given up direct rule in the 15th century, but still exerted great influence and, at various times, took what amounted to tribute and/or tried to regain direct control of the area.
During the 19th century, France decided it wanted Vietnam as a colony, primarily for financial reasons, though also because Catholic missionaries wanted to save souls there. They fought a series of battles, first in the south, and then in the north, defeating the locals and Chinese mercenaries (who were operating with their government's blessing and maybe money, but I digress . . .). They did this with relatively small numbers of troops; the battles that took place involved only a few hundred to a few thousand men.
Why Vietnam? Besides saving souls, French businessmen got raw materials and rice real cheap. But the colonial administration was less than enlightened - torture, concentration camps, the works - and the government had to deal with various rebellion movements right up until World War II, when the Nazi conquest of Vietnam made Indochina a de facto Japanese colony, a key base during the Asian portion of the war.
So why were the commies so strong there? Two reasons: Because the communist party provided the structure and discipline necessary for an anti-French movement to survive the repression, and China (and Russia) supplied aid to their fellow travelers. When the war ended, the communist-dominated liberation movement led by Ho Chi Minh quickly moved to seize the country. The liberated French government actually concluded a treaty with Uncle Ho after the war, which effectively would have turned control of the country over to a national government, but it was clear that the two sides were working with different aims -- the French wanted Vietnam back as a colony, and Ho wanted to establish a communist state. Things quickly came to a head . . . and every other body part.
* If I call them colonies, you're going to think of America, and that's not really a good parallel. And yeah, I'm just going to gloss over the whole protectorate/colony thing, which distinguished different parts and times of the administration. The French arrangements were even more complicated than the British, but then that's France for you.