Beinart makes that clear:
Given the overwhelming evidence, both from politicians and the public, that isolationism in America today is virtually nonexistent, why do so many high-profile commentators and politicians depict it as a grave threat? One clue lies in a word that these Cassandras use as a virtual synonym for isolationism: “retreat.” If the subtitle of Bret Stephens’s forthcoming book is The New Isolationism and the Coming Global Disorder, its title is America in Retreat. In their op-ed warning of a new “cycle of American isolationism,” Lieberman and Kyl employ variations of “retreat” or “retrench” six times.
But “isolationism” and “retreat” are entirely different things. Isolationism has a fixed meaning: avoiding contact with other nations. Retreat, by contrast, only gains meaning relatively. The mere fact that a country is retreating tells you nothing about the extent of its interactions overseas. You need to know the position it is retreating from. Herein lies the rub. In general, the isolationism-slayers are far more comfortable bemoaning American retreat than defending the military frontiers from which America is retreating. That’s because those frontiers, which reached their apex under George W. Bush, were both historically unprecedented and historically calamitous.