“Remember when they said that U.S. Troops wouldn’t fire on fellow citizens?”
Lincoln had no problem with recruiting soldiers who would do that. All you have to do is demonize fellow citizens with labels that make them less than human, such as “rebels” since if the government had said they were citizens of another nation that would have undercut their argument against the validity of secession.
Today, it would be “facists” or “deplorables” or “MAGA” extremists or words like that.
The Romans justified the killing of Christians by labeling them as enemies of mankind.