Left-wing activists smash windows with bats to protest conservative speakers. They burn property and threaten opponents in public, violently lashing out at individuals for holding conservative views. It’s 2019, but could be mistaken for 1969. That’s no accident, observers of history say. The roots of disorder go back to a style of agitation and organization made famous half a century ago by left-wing activists. Because it so often worked for the Left, it has become common in mainstream politics today, and has even been copied recently by a few on the Right. Kenneth Starr, the former federal judge and solicitor general, discussed one of the most notorious cases in which radicalization moved out of the realm of theory into practical, aggressive politics. Addressing the Washington Examiner’s first annual political summit, at Sea Island, Ga., in November, Starr recalled the 1969 college graduation of Hillary Clinton and her affection for the philosophy of community organizer Saul Alinsky, who advocated personal targeting by radical activists in the 1960s and ’70s. Alinsky was unhappy with what he saw as left-wing academics’ and politicians’ timidity and acceptance of political norms. He tried to organize businesses, labor unions, churches, and neighborhoods into radical activists who could force swift change with direct action that circumvented the traditional political process. Starr recalled Clinton, who went on to be first lady, senator, and secretary of state, giving the first ever student speech at...