Part of it was that both the US and RVN were signators on the SEATO Treaty. This was the SouthEast Asia Treaty Organization, and was similar to NATO. There were several treaties at that time, all with the intent of stopping and containing the spread of Communism. There was also the Tonkin Gulf Resolution, which was passed by Congress in the very early sixties, during the Kennedy Administration. Kennedy sent in the first advisors, Johnson committed the rest of us, and Nixon finally ended it. That's it in a nutshell.