Early in his maiden speech to the United Nations General Assembly on Tuesday, President Donald Trump offered fellow world leaders the best, most concise summary he’s ever provided of his approach to world affairs: “We are guided by outcomes, not ideology,” he declared.

Having signaled that his listeners should be prepared for some blunt, hard-nosed pragmatism, Mr. Trump proceeded to deliver just that. In many ways, in fact, Mr. Trump’s address marked the return of American foreign policy to realpolitik: a set of principles...