worth pondering when considering a war of aggression. Unless a conquered population is exiled or annihilated, you create enemies for your grandchildren to deal with.
Thus was the Union "preserved." As is obvious but also sometimes profoundly learned in romantic relationships, sometimes to love someone means having to let them go. One of the ironies of the Civil War is that Lincoln fought the war because the Southerners 'wouldn't let the slaves go.' He should have led by example and let the Southerners go.