This week marks the 157th anniversary of the Battle of Gettysburg. As it happened, Union victory in Pennsylvania on July 3, and the surrender of Confederate forces at Vicksburg, Mississippi, on July 4, portended a gradual death for the Southern rebellion. When the Confederacy collapsed in 1865, so, too, did the foundation of its apartheid society: institutionalized racial slavery. Yet the ultimate success of Union arms was hardly a foregone conclusion in 1863. Emboldened by its victory at Chancellorsville, Robert E. Lee’s Army of Northern Virginia crossed the Potomac River, passed through Maryland, and raided south-central Pennsylvania. It plundered farms,...