It was a different time. Communism was hell bent on world domination. The Cuban missile crises was proof enough.
Soviet Russia was capable and willing to do considerably more damage than lighting their shoes on fire or neutering themselves with an underwear bomb.
Johnson was faced with some extremely bad choices. He could allow the spread of Communism and risk a nuclear war as a final outcome. Or he could take a course of action he knew was wrong.
Lyndon Johnson was not interested in waging a war in Southeast Asia. In fact, he was desperate to avoid a conflict that might compete with the funding requirements for his expansive domestic agenda. His national security team, primarily composed of Defense Secretary Robert S. McNamara, National Security Advisor McGeorge Bundy, and Secretary of State Dean Rusk strongly urged or at least did not dissuade the President from pursuing an aggressive stand against North Vietnam. These advisors were concerned that if the U.S. did not respond to North Vietnamese aggression, the nation would lose credibility in its dealings with the Soviet Union, China and other communist threats.
Yes, it was the wrong thing to do, but their were far fewer options than their are currently in our optional war in Syria.
Should there be any wonder, I was on the side singing songs and carrying signs.
Johnson likely could have won a second term. He chose not to run. Not only did he loose the nation, I believe he no longer wanted the job.
http://www.informationdissemination.net/2013/09/choosing-war.html