In the segregated American South, policing was war. Ungovernable police discretion came to the backroads and cattle pastures of America’s rural countryside as ideas of race, property, and belonging reshaped state power. In Mississippi Law, Justin Randolph explores policing’s hinterland to explain US racial authoritarianism between the Civil War and the civil rights movement. In Jim Crow Mississippi, the force behind the police officer’s autocracy carried legacies of empire and slavery into the age of agribusiness and automobiles—from state troops and slave patrols to state troopers and highway patrols. But this was no isolated story of individual barbarism. US military and reform traditions informed ruling-class beliefs in thoughtful police improvement, through both the state militia and its inheritor, the state police. Black Mississippians fought to raise awareness for and defend their loved ones against the violence spawned by paramilitary police reform. Some took up arms against police officers; some imagined a legal off-ramp to remake public safety after Jim Crow. Ultimately, the Americanization of what one activist called “Mississippi Law” came with more funding and more authority for policing, a key piece of infrastructure for the age of mass incarceration that followed the civil rights revolution. Recounting the work of famous and forgotten activists, Mississippi Law is a genealogy of Jim Crow rule and dreams of a safety that might have been and might yet be.