Regarding the Vietnam War, what were the United States' interests or reasons for becoming involved in the war?

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.