By the fall of 1854, most Americans realized that their nation’s protracted effort to avoid open conflict over slavery was failing. The passage of the Fugitive Slave Act in 1850 and the publication of Uncle Tom’s Cabin two years later had riled northerners and southerners in turn, and the armed struggle between proslavery and antislavery settlers in the Kansas Territory was just beginning. While many of the nation’s political and cultural elites were still keen to avoid the topic of disunion, the directors of the Historical Society of Pennsylvania steered into the storm. That November they invited John Latrobe, a prominent lawyer from Baltimore, to deliver their annual public lecture. His subject was “The History of Mason and Dixon’s Line.” Stretching west for nearly 250 miles from the Chesapeake through the Appalachians, the line had been drawn between 1763 and 1767 with unprecedented precision by two English surveyors, Charles Mason and Jeremiah Dixon. Originally intended to settle a boundary dispute between Pennsylvania and Maryland, by 1854 it had assumed an ominous new meaning as the …