What event in february of 1968 turned american public opinion against the vietnam war?

The correct answer is the Tet Offensive.

This military attack by the North Vietnamese on the Southern Vietnamese and the US displayed how the US was not having success in the Vietnam War. The lack of progress made by the US in the Vietnam War was summed up well by Walter Cronkite, one of the most famous journalists in America during this time. After covering this story, Cronkite discussed how it looked like America's involvement in Vietnam was not having success and that this war would probably end in a stalemate.

These words by Cronkite hit the American public hard, causing many of them to change their mind about America's involvement in the Vietnam War.