‘ARE WE ROME?” asked the author Cullen Murphy a decade ago, in a provocatively titled book that compared the 21st-century United States with the final days of the Roman Empire. Lately there’s been no shortage of Cassandras who emphatically answer yes. “The speed with which we’re recapitulating the decline and fall of Rome is impressive,” opined the conservative commentator Bill Kristol, in a tweet this past July, taking issue with President Trump’s treatment of the press. “What took Rome centuries we’re achieving in months.” And the Cornell historian Barry Strauss bemoaned the parallels between alleged American failings and those of...