The year 1968 was a big turning point in the Vietnam War, because L.B.Johnson had increased its commitment to S.Vietnam. In February 1968, the Vietcong launched the Tet Offensive against S.Vietnam and US; also, it caused US withdrawal indirectly. What made the Tet Offensive so important?
Politically, at the begging of the war, the US government thought that they would absolutely win in the Vietnam War because America was a world superpower all the time, also they have more powerful weapons and more supply. Achievement also gave them a confidence. The US always had won the War, for example WW1 and WW2, except Korean War in 1960-1963. In 1996, Philip Caputo the solder of US Marines in Vietnam says "but sometime later on we got this idea that the United States was invincible. Not unjustifiable, we thought, because we'd never lost a single war we'd ever fought. I think we'd fought 12 or 13 wars before then and we'd won every one, including the one against ourselves.
"from this statement, clearly we can see the inattention of the US.
However, in February 1968, Tet, the Vietnamese New Year, the US troops had received surprise attack from Vietcong and National Liberation Front (NLF). The commander of U.S. forces in Vietnam Gen. William Westmoreland described it, says
"The American publics were caught by surprise. We were making military progress at the time -- which [is] a statement of fact. And when the Tet Offensive took place, the American people were not prepared for that, and I assume some significant responsibility for that. And I've made this statement many times. If I would have to do it over again, I would have made known the forthcoming Tet Offensive. At that time, I didn't want the enemy to know that I knew...