From the heart of the Midwest to the Deep South, from the mountains of North Carolina to the Texas frontier, words like these have echoed through more than a century of American history. The call heralded not a tornado or a hurricane, but a very unnatural disaster: a manmade wave of racial cleansing that purged black populations from counties across the nation.

We have long known about horrific episodes of lynching in the South, but the story of widespread racial cleansing above and below the Mason-Dixon Line has remained almost entirely unknown. Time after time, in the period between Reconstruction and the 1920s, whites banded together to drive out the blacks in their midst. They burned and killed indiscriminately and drove thousands from their homes, sweeping entire counties clear of blacks to make them racially "pure". The expulsions were swift; in many cases, it took no more than 24 hours to eliminate an entire African-American population. Shockingly, these areas remain virtually all-white to this day.