At a Munk Debate in 2018, Canadian psychologist Jordan Peterson stumped his opponents by asking what should have been an easy question: “When does the Left go too far?” America has rightly developed a heightened sensitivity to warning signs of radicalization on the Right. The unspeakable legacies of the Nazis and the Hutus, among other terrible right-wing regimes of history, have made us responsive to the slightest hints of racial superiority and malignant nationalism. But when it comes to the pathologies of the Left, the lines are stubbornly blurry. History is replete with malevolent left-wing governments that caused unfathomable suffering,...