The European Union, optimists point out, did not unravel after Britain’s departure. These days, the political talk in London is about ways in which Britain can draw closer to its European neighbors. Still, that sense of possibility has come only after years of turbulence. Economists expect similar chaos to buffet the global trading system as a result of Mr. Trump’s theatrical exit.

“It will not be the end of free trade, but it is certainly a retreat from unfettered free trade, which is the way the world seemed to be going,” said Eswar S. Prasad, a professor of trade policy at Cornell University. “Logically, this would be a time when the rest of the world bands together to promote free trade among themselves,” he said. “The reality is, it’s going to be every country for itself.”

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