Regular folks and history buffs who believe Maryland leaned strongly toward the Confederacy during the Civil War era have never lacked evidence for the claim. It was a Marylander, after all, on the U.S. Supreme Court who wrote the opinion in the infamous 1857 Dred Scott case, which found that Black people were not citizens — a ruling that helped spark the fighting. And Marylanders voted for a Southern sympathizer, not Abraham Lincoln, for president in the election of 1860. Then, some 20,000 Marylanders took up arms for the Confederacy. But such facts can be deceiving if looked at in...