How did public protest cause the American withdrawal from Vietnam?

Following the 1968 TET offensive President Johnson said the public had decided the Vietnam War was not winnable. Johnson announced he would not seek a second term in the 1968 presidential election. Between 1968 and 1973 the Vietnam War kept grinding on, but there was little to no progress after Nixon was elected in 1968. In fact the war spread to Laos and Cambodia, but Watergate in 1972 finally got to Nixon, and in 1973 the U.S. cashed in her chips as far as Vietnam was concerned, it (the Vietnam War) was not worth the fight in terms of American lives and money.