As President Donald Trump flies to Europe for a week of summitry, the trepidation in European diplomatic circles is palpable. With trans-Atlantic relations at their lowest ebb in postwar history, one question has dominated discussion this year in Europe’s chancelleries and corridors of power: Is Mr. Trump’s goal to reform the multilateral system that has underpinned the West’s security and prosperity for the last 70 years? Or does he want to destroy it?
There is no question that the scale of Mr. Trump’s assault on the institutions...