The public story that the public was given was broached in the domino theory.  Many in the American Government of the time genuinely believed that the landscape was divided between nations that were democratic and those that were Communist.  The fear that gripped most post Cold War American governments was that if one nation in a region fell to the Communist, it would only be a matter of time when all the nations in that region would do the same.  The fundamental argument here became that if Americans believed that democracy was a conviction that was worth defending and exalting, then it must show the will to face down its adversaries at any juncture or nation.  This became the rationale behind why American intervention in Vietnam was needed.  The perceived fear of the North Vietnamese Communist government receiving support from the Russians and Chinese and running amok all over South East Asia struck a note of fear in American policy makers and this provided the framework for why American involvement in the Vietnam conflict became a binding commitment.

The reason that the US wanted to get involved in the Vietnam War was that the US wanted to prevent the spread of communism.

The US believed in a theory called the "domino effect."  This theory argued that, if South Vietnam feel to communism, other countries in the region would become communist as well.  When this happened, key American allies such as Japan and the Philippines would have been threatened.  This would reduce the security of the US.

So the basic reason for US involvement was the US Cold War strategy of containment.