Question

What is the main reason the United States backed South Vietnam?

Answer 1

The US was trying to follow the "Policy of Containment", in which they try to contain or even push back the spread of Communism. This is the main reason, for South Vietnam did not have:
#1: a good reason for war 
#2: No good military defense or offense area.
#3: no good support from the public

#3 is the largest reason that the US lost, for the US usually only won when they had wide-spread support from it's people.

hope this helps

Answer 2

The causes of the Vietnam War revolve around the simple belief held by America that communism was threatening to expand all over south-east Asia. Neither the Soviet Union nor the United States could risk an all-out war against each other, such was the nuclear military might of both.