In 1968, the Vietcong launched a surprise offensive during the Tet (New Year) festival. They attacked 36 cities and even reached Saigon where they held the US embassy. They were eventually forced to retreat with very heavy losses. From now on, it was the North Vietnamese army that did most of the fighting, rather than the Vietcong.
This was a military victory for the USA but, in the long term, it had a disastrous effect on the attitudes of Americans, especially witnessing their own embassy under attack, as well as the thousands of civilian casualties and thousands of refugees. Opinion was also turning against American involvement in Vietnam.