The Vietnam War faced mass opposition for several reasons inlcuding:
1) Unfair draft practices- Many citizens who were affluent and had connections were able to avoid being drafted by the US military. This resulted in a majority of the soldiers being individuals from lower-middle class socioeconomic groups. Many citizens felt that this was unfair, an dubbed Vietnam a rich man's fought that happened to be fought by the poor.
2) Monetary cost- The Vietnam War cost America billions of dollars over the course of the 2 decades of their involvement in trying to keep South Vietnam out of the control of the Soviet Union/communism.
3) Casualties- The US military suffered over 50,000 deaths due to this war. Americans felt that this was unacceptable.