The previous post is very lucid as to this point. I think that the perception of the conflict might reveal much about individual beliefs on this question. Those who felt it was a pure civil war would likely believe that there is no reason for any other nation's intervention into such a situation. At the same time, those who felt that there were larger implications to the struggle in terms of assessing the reality of a Communist South East Asia would feel that the conflict represented more than a mere civil war. I think that one can say that the conflict embodied a moment where both civil war realities were present which converged with the implications of post- Colonial and Cold War fears and predicaments.
North Vietnam was communist and South Vietnam was non-communist. Communist countries supported North Vietnam while non-communist countries supported South Vietnam.
The war was more than a civil war between this divided country. There were factors involving other nations that started the war. Between 1945 and 1954, Vietnam had waged an anti-colonialism war against France, and in turn the United States gave Vietnam over two and a half billion dollars in support. It was the intention of France to restore its colonial dominion after WWII. In 1954, Vietnam gained its independence from France and consequently was temporarily divided into North Vietnam and South Vietnam. South Vietnam refused to attend unification elections and the United States supported them.
So all of these events led to the conflict.