Last summer, after generations of silence and denial, the city of Tulsa, Okla., began excavations in search of the graves of victims of the 1921 Tulsa Race Massacre. Nearly a century had passed since a mob of white Tulsans violently invaded the prosperous Black neighborhood of Greenwood, overcoming a vigorous defense by some residents, killing and rounding up Black Tulsans en masse and burning much of the neighborhood to the ground. The search...