What does the lable "copperhead" have to do with the Creightons in Across Five Aprils?

During the Civil War, there were a lot of people in the North who did not approve of the war.  Many of them believed that the Southern states should be free to have slavery if they wanted.  Many of them hated black people and disliked the war because they thought that the war was going to help black people.  The derogatory term for Northerners who disliked the war was "Copperheads."

The Creightons are sometimes referred to as Copperheads by people in their area because one of the Creighton boys has gone off to fight for the South.  This makes it seem that they are Confederate sympathizers.