The roots were found in pacifist beliefs and sometimes even in religious beliefs. People believed that the government was being corrupt and lied about the war motivation. As intellectuals opposed the war, they were soon joined by students who created groups like Students for a Democratic Society, who were instrumental in the antiwar campaign.
The antiwar movement grew from earlier groups such as Quaker and Unitarian peace activists, SANE, the Old Left, and socialists. Students also learned from their experience in the civil rights movement. Students for a Democratic Society (SDS) at the University of Michigan and the Free Speech Movement at the University of California at Berkeley were the two main student groups at the start.
The main way in which the US involvement in the spanish-american war represented a shift from isolationism to intervention is that this was the first time the United States had really committed troops and resources to a war that had seemingly no effect on the expansion of US borders. It was more done out of altruism.
