During the Vietnam war if we went in there to actually defeat the enemy then why were we there?

When we tried to liberate North Korea twenty years earlier we sucked the PRC into the war and it determinate into a stalemate that went on for the better part of three years after all the shouting was over and we both should have signed a cease-fire. Kennedy/Johnson and their main adviser Robert McNamara were afraid that if we took the war to North Vietnam with ground troops the Chinese would likely intervene and spread what we were trying to keep a regional war into a major World War. The felt we were pushing the envelope by just bombing North Vietnam. But they also extended their fear to the borders of the supposedly neutral countries of Cambodia and Laos. In fact the Communists involved both those countries early on, and they were NEVER neutral. The only ones pretending that they were, were the Americans. Johnson/McNamara ordered the American military to observe the border areas of these countries as neutral and allowed the Communists to use them as samurais in which they could build large supply bases and rest areas for their troops that could not be touched. In the end the Communists used these bases as launching points for major attacks on South Vietnam that could, and should, have been avoided and which finally brought the government of South Vietnam down. I can accept the rationale for not invading North Vietnam, but do not believe China was about to go to war over the borders along the South Vietnamese western border. The Communists SHOULD have been taken out. It would have probably set up a situation where the North could not have sustained its war against the South.