The above post is very accurate, and I have some additional thoughts. General William Westmoreland, the commander and public face of the Vietnam War on the evening news, had told us in early January 1968 that enemy attacks had dropped off to nothing, and that the "Light was at the end of the tunnel". There was talk about most troops coming home by Christmas of that year. Remember that by this time, the war was almost four years old for us, and had the complete faith of the vast majority of Americans.
Enemy attacks had dropped off to nothing because the Vietcong and North Vietnamese were preparing for the Tet Offensive. It took us completely by surprise, during a holiday cease fire. They attacked at every major US base in the entire country, the North Vietnamese Army rolled across the border and occupied the ancient city of Hue, and VC sappers even occupied part of the US Embassy in downtown Saigon, where it was supposedly safe.
Americans saw all of this on their television news at night. They felt duped. They felt like after four years of war and sacrifice, our leadership was unaware of the real situation. Walter Cronkite, the most influential man in TV news, gave an editorial saying the war was "unwinnable". President Johnson, upon hearing this, said "If I've lost Cronkite I've lost middle America!" He decided not to run for re-election.
For the first time, a majority of Americans said they were against the war, and for the next five years, the government never won the peoples' confidence back.