There were a number of reason that why did many Americans oppose US involvement in the Vietnam War, however public knew US had to stop the spreading of communism.
First of all, one reason many Americans oppose US involvement in the Vietnam war is that Vietnam War was the first war reported live coverage by many medias. So when Americans are having a breakfast, the war is keep going at the opposite of earth and they could see the shocking movie. For example, on March 16, 1968 The My Lai Massacre happened. It was a massacre by U.S. soldiers of three hundreds to five hundreds of unarmed Vietnamese civilians, mostly women and children, on March 16, 1968, during the Vietnam War. Becoming a symbol of US-American war crimes in Vietnam, it prompted widespread outrage around the world and reduced public support for the war in the United States. In addition, on February, 1968 Tet Offensive occurred.
However, America beat back NLF, it received as American's defeat. Because, media reported about Tet Offensive, and as a result, after the Tet Offensive, US anti-war movement got violent. Media can give lots of people information; so actually, media had huge influence on the anti-war movement on the Vietnam War.
Secondly, US government spent too much money on the Vietnam War. From 1953 to 1957, US government gave $ 823.3 million to South Vietnam as a economic aid and $ 277.8 as a military aid. In addition they gave $900 million as an economic and military aid in a year. According to my resource, US government had to spend $2000 million for keep going fight. If it's practical, may be people didn't complain about it. US Dropped three times number's of bombs compare with WW2 and they also had used new weapons, such...