Decline, not the Donald, is the specter haunting Europe today. The numbers make this clear. Some readers objected to World Bank data in last week’s column showing that, in dollar terms, the eurozone economy had not recovered from the 2008-09 financial crisis. In euro terms, they point out, eurozone gross domestic product has been growing. But even using the euro-denominated figures issued by the European Central Bank, the growth rate from 2009 to 2017 was only 0.6% per year. That’s anything but robust.

Mr. Trump’s tweets aren’t the reason populists are governing Italy and the gilets jaunes revolt has shaken Emmanuel Macron’s reform efforts in France. Since the financial crisis, the Italian economy—measured in euros—has been shrinking at an average rate of 0.5% a year, while the French economy has been growing at an average of only 0.8%. Only the gush of cheap energy made possible by American fracking keeps these fragile economies afloat; at an oil price of $125 a barrel, the eurozone—and its banking system—might well face another economic crisis.

Asked about the importance of European unity to the U.S. in January 2017, President-elect Trump told the Times of London: “I never thought it mattered. Look, the EU was formed, partially, to beat the United States in trade, OK? So I don’t really care if it’s separate or together, to me it doesn’t matter.”