It’s disturbing to think that this modern Moloch might have exercised so much influence over midcentury America, as Mr. Bird maintains, with Cohn’s baleful legacy extended into the present day by President Trump. But is this a plausible claim? ... While Cohn’s historic significance is, at best, minimal, his status as a menacing specter in the minds of liberal journalists has only grown. Which makes it especially regrettable that, while Mr. Bird offers considerable detail chronicling Cohn’s colorful legal career and enduring interest to gossip columnists and magazine profilers, all the entertaining stories and lurid set-pieces are spoiled in the service of an underlying thesis—Cohn as linchpin of modern conservative politics and Trump’s Svengali—which grows less persuasive with more repetition.