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.