BARCELONA, Spain — In what might be seen as a Memorial Day gift to President Trump, residents of a mountain range outside of El Paso today woke up to a new vista—a mile-long wall of metal slats constructed along the border with Mexico that just shot up over the weekend. The secret project, which started on Friday evening and was completed by Monday morning, was the work of We Build the Wall, a new group whose director is the former White House strategist Steve Bannon. The CEO is Air Force veteran Brian Kolfage, who had raised $20 million to build...