Near the end of 1861, with the American Union crumbling, President Abraham Lincoln became obsessed with an unusual document. Nearly three feet in length, it appeared at first to be a map of the southern states. But it was covered with finely rendered shading, with the darkness of each county reflecting the number of slaves who lived there. South Carolina, the first to secede from the Union, featured a particularly dark coastline. Yet other parts of the South (like western Virginia) appeared as islands of lightness.

Lincoln often studied the map, and it “bore the marks of much service,” according to a memoir by Francis Bicknell Carpenter....