It's my understanding that the politicians in that era were concerned with communist expansion in Southeast Asia and were of the opinion that it was better to stop it there, Vietnam, before it consumed the whole area and possibly spread to Australia and the surrounding area. Remember that the build up in Southeast Asia, most notably Vietnam, occurred shortly after the Cuban missile crisis and the fear of communist aggression was at it's zenith. Right or wrong, I would much rather fight on someone else soil as my own.