Violence has been a political tool of the Left at least since revolutionaries introduced the guillotine in late 18th-century France. The United States didn’t experience this kind of destructive, sanguinary urge during the War of Independence, though it increasingly does now. What was it that injected this modern strain of radicalism and violence into our politics?  Perhaps no one helped bring violence and the disruption of all norms into the mainstream from the fringes more than Saul Alinsky. His book Rules for Radicals explains what we are seeing today and how we got here. Just as the title of this 1971 work of practical militancy suggests, it codified tactics intended to overturn the constitutional order. At the same time, the “rules” Alinsky had championed threatened — and promised — to propel American politics ineluctably into violence. The most reprehensible accomplishment of Rules for Radicals was convincing a generation of progressive Democrats that it no longer needed to ask the fundamental question, “Does the end justify the means?” Indeed, Alinsky derided this broad rule of civilized conduct as a “meaningless” moral formulation. The only query worth posing, he said, was, “Does this particular end justify these particular means?”  Saul Alinsky on Chicago’s south side, Feb. 20, 1966. (Associated Press) Alinsky fashioned himself a patriot, but that is highly dubious, and his view of politics as a consequentialist venture stripped of personal or national principles was deeply...