The shift in American attitudes “toward a post-imperial role” began before Mr. Trump, with the failure of the Iraq war, noted Nathalie Tocci, director of Italy’s Institute of International Affairs and an adviser to E.U. foreign-policy chief Federica Mogherini.

But for her, “the silver lining in Europe is that even the current dodgy leaders realize we’re all very small.”

“There is a growing realization that a stronger Europe and European Union are a necessity, whatever the faults,” she said.

Daniel W. Drezner, who teaches international politics at Tufts University’s Fletcher School, argues that “Americans are sick of wars in Afghanistan and Iraq,” but that both Mr. Kagan and Mr. Walt are wrong about American public opinion.

“Ask them about trade, immigration and alliances, and it turns out that Trump has made liberal internationalism great again,” he said, with Americans favoring international trade and alliances with European and Asian democracies.