In 1838, Rep. William Graves, a Kentucky Whig, shot and killed Rep. Jonathan Cilley, a Democrat from Maine, and became the only member of Congress to murder one of his colleagues. The events that led to the fatal duel were convoluted and—even by today’s standards—almost breathtakingly stupid, but in her new book, Field of Blood: Violence in Congress and the Road to the Civil War, Yale historian Joanne Freeman distills the incident to its essence. The duel, and others like it, was both a reflection of and a driving force behind partisan and sectional divides that would ultimately cleave the nation in two.

The product of 17 years of research and writing, Field of Blood tells you something you don’t know about something you may have thought you knew—the nation’s long march to the Civil War. Though a few incidents, such as the Graves-Cilley duel and the 1856 caning of Sen. Charles Sumner, have lingered in the history books, the full scope of antebellum violence in Congress has gone untold. Poring through old letters and diaries, Freeman found more than 70 incidents of violence—or threats of violence—between 1830 and 1860. And the violence served a purpose: Threats and brute force were tools by which the Southern slave power exerted its will on the federal government—until the North began to fight back.

Freeman spoke with Mother Jones in December about the book, historical memory, chaos in Parliament, and a man with the nickname “Extra Billy.”