One hundred years ago, on November 25, 15 men climbed atop Stone Mountain, just outside Atlanta, touched a lit match to a kerosene-soaked cross, and resurrected a terror from America’s past.

The Ku Klux Klan, dead for some 40 years, was back.

Their mission? To defend white, Protestant, native-born America from “illegal foreigners” and religious minorities. Faced with unprecedented ethnic and cultural change, at least 3 million Americans—South and North—responded by joining a violent, secretive movement, vowing to keep America from becoming …