What are some of the different groups that opposed the Civil War?

Question

why did they oppose it?

Answer 1

1. conscientious objectors (pacifists vs. ANY war), usually for religious reasons - such as the Quakers

2. "Peace Democrats" or "Copperheads", who blamed abolitionists for the war (worthy of special note - Clement Vallandigham of Ohio)

3. anti-draft groups - many Irish immigrants among them

4. Unionists in the South, who had opposed secession (strong in the mountains of Appalachia) and some of whom organized "peace societies"

Answer 2

are you talking about opposing the war or slavery? the slavery, well, the free will baptist denomination did, though some of the ministers remained neutral.