Why do the drunken horseback riders call Matt and Bill "Copperheads"?
From the Union state of Illinois, young Bill Creighton makes the difficult decision to go and fight for the Confederacy, despite the fact that his brothers are fighting for the Union. Father Matt Creighton will not condemn him. The drunken horsemen leave a message in Chapter 6, emphasizing their feelings towards Northerners with Southern sympathies, those they call "Copperheads" - "Theres trubel fer fokes that stands up fer there reb loving sons."
"Copperheads" was the derogatory slang term for a Northern person with Southern, anti-Union sympathies