OAKLAND, Calif. — Last weekend, when a 27-year-old bike messenger showed up at the “Unite the Right” rally in Charlottesville, Va., he came ready for battle. He joined a human chain that stretched in front of Emancipation Park and linked his arms with others, blocking waves of white supremacists — some of them in full Nazi regalia — from entering. “As soon as they got close,” said the young man, who declined to give his real name and goes by Frank Sabaté after the famous Spanish anarchist, “they started swinging clubs, fists, shields. I’m not embarrassed to say that we...