President Trump got bad advice about the secession crisis in Catalonia. When Spanish Prime Minister Mariano Rajoy visited the White House last week, he said that Spain’s northeastern region would be “foolish to secede.” On the contrary, the gutsy Catalans are the world’s poster-boy for populism. Their independence movement is a real revolution. America shouldn’t meddle in Spain’s internal affairs, to be sure, but we ought to recognize a kindred political movement when we see it. Ultimately, the Catalan independence movement is a response to Europe’s demographic cataclysm.