There was a large contingent of individuals in the North known as "Copperheads," who genuinely opposed a violent reunification of the nation for various reasons. These tended to revolve around accusing the Lincoln administration of sacrificing civil liberties, accusing the government of waging an abolitionist war and/or criticizing military measures that resulted in high casualty rates. Pro-war citizens tended to bemoan these individuals as treasonous, with Federal, military and even civilian elements working towards silencing these anti-war protesters. The arresting of Ohio Representative Clement Vallandigham is a famous example, although there are other well documented examples, such as the arresting of the Maryland Legislature in 1861 and the trials of John Merryman. Issues surrounding the suspension of Habeas Corpus are also strongly associated with mitigating dissent and attempting to ensure a relatively smooth prosecution of the war.

Historians have tended to see the Copperheads, however, as naïve idealists who believed the nation could be reunited without war. So they tended to be Unionists, but demanded non-military solutions that were never viable and tended to ignore evidence to the contrary. If you are super interested, here are few book recommendations on these various topics:

For Copperheads in general, see Jennifer Webers, “Copperheads: The Rise and Fall of Lincoln’s Opponents in the North.” For issues concerning treason and civil liberties, Mark Neely’s “the Fate of Liberty,” and William Blair’s “With Malice Toward Some” are good places to start. On what motivated Northerners to go to war and vigorously oppose the Copperheads, I would recommend the following: Earl Hess, “Liberty, Virtue and Progress,” James McPherson, “For Cause and Comrade,” and Gary Gallagher “the Union War.”