While it wasn't necessarily a good idea, going to war in Vietnam was the most logical course of action.
We'd already been supplying weapons, vehicles, and advisors to South Vietnam while the French were still at war alongside SV. In fact, we were contributing MORE in terms of hardware than the French were themselves. We already had so much in place, all we needed to do to continue the war ourselves was send troops in.
The big thing though was Communism. We did not want Vietnam to turn Communist. China, Vietnam's next door neighbor, had already announced its status as a Communist nation some time ago, and we feared a domino affect. What was next? India, Indonesia, the Phillipines, Japan? We viewed Communism, and more importantly, the nations and leaders that supported it, as a threat.
So, we did what we could to stop it. And when conventional tactics weren't working, we resorted to doing some pretty nasty stuff. It's tough, but hey, that's war.