The Signal messages appear in staccato bursts: “We all in blac bloc bout to join.” “We in the streets.” Looking up from my cellphone, I see them coming: a platoon clad head to toe in black, marching in formation like some sort of dystopian Roman legion. They’re carrying flags and holding shields, some embossed with the black-and-red logo of the anti-fascist movement. As they approach the column of protesters already marching toward Berkeley City Hall, they start banging on their shields, chanting: “Ah – anti! – Anti-fascista! Ah – anti! – Anti-fascista!” Hundreds in the main crowd – from small...