COLLEVILLE-SUR-MER, France—At a time of political upheaval across the European Union, it was President Trump who was the relative picture of stability—both diplomatically and politically—as he skipped back and forth across the English Channel this week.

For all of Mr. Trump’s troubles back home—his approval remains stuck below 50%, Democrats are increasingly supportive of impeachment proceedings and Congress frequently deadlocked—he’s watched Europe change while he has mostly stayed the same during his 28 months as president.

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