Few people in the North welcomed South Carolina’s secession in December 1860, but Frederick Douglass, America’s most prominent former slave and African-American abolitionist, was one of them. From his editorial desk in Rochester, N.Y., Douglass heaped scorn on the Palmetto State’s rash act, but he also relished it as an opportunity. He all but thanked the secessionists for “preferring to be a large piece of nothing, to being any longer a small piece of something.” ...