Considering that the Vietnam War, which lasted from 1945 to 1975, still stands as the most protested war in American history, it is important to understand why people protested the Vietnam War.

Many historians agree that there are three principal reasons people were so opposed to the Vietnam War and that these had more to do with the technology and recent history than any real fervent feelings against violence (although the movement did evolve to be mainly about peace):
The Vietnam War was extremely real to those in the U.S. For the first time, thanks to the development of media, the American population had direct access and exposure to the devastation that was going on in Vietnam.
The violence and bloodshed of war was so much more real to the public and this intensified fear, sympathy and outrage. People could see and hear the soldiers, the gunshots and bombs, the dead bodies. War looks a lot less heroic when you are a part of the action and you get an unaffected sense of the fear and the misery involved. Needless to say, the vicarious exposure to war did not sit well with the general public.
Secondly, fear of communism was slowly dying out and many people did not feel that the anti-communist basis for U.S. involvement in the Vietnam war was enough. It just did not make logical sense to risk the lives of thousands of American soldiers in an effort to keep South Vietnam from a communist take-over by North Vietnam.
The United States argued a domino effect theory for their involvement in Vietnam, but the majority of the American public had very little fear of communism reaching the U.S. For many, the war was simply not morally, legally, or rationally necessary.
Lastly, and perhaps most importantly, many people opposed the war because of the draft. Lower and middle class citizens were especially threatened by the prospect of forced conscription and so they voiced protests.
Furthermore, recent activism relating to the free speech movement and the civil rights movement inflamed anti-war protests. Liberalism became the way and people took advantage of a new-found power through protests.
On a grand political scale, the United States became involved in the Vietnam war (which was principally a war between communist North Vietnam and the government of South Vietnam) in an effort to halt the spread of communism.
Although military advisers began going over to Vietnam in the 1950s, the United States involvement did not become significant until the 1960s. Opposition to the war began in the 1950s, but it was only a small percentage of Americans who were either left-wing supporters wanting a North Vietnam victory, pacifists, or liberals who did not agree with the authoritarian South Vietnam government either.However, by the 1960s, the anti-war movement had grown exponentially and it came to characterize that period.
The first march to Washington D.C in protest of the Vietnam War took place in December 1964. There were 25,000 people involved, making it the largest anti-war demonstration in American history.
As the years wore on, several anti-war movement groups were created and hundreds of anti-war demonstrations took place. Students were at the forefront of the anti-war movement as they were not drafted into war, and therefore free to voice opinions from the home-front.
In addition to this, thousands of drafted men refused their draft orders (the most famous of which is Mohammad Ali) and even more soldiers deserted (apparently over half a million members of the U.S. armed forces). The war itself was extremely expensive, costing about $66 million a day by 1968. This, of course, was another point of contention for the protesters. Many people were asking why the U.S was wasting money and other valuable resources on an unnecessary war.
The anti-war movement against the Vietnam War defined an entire generation and changed the course of American history.