I watched them on a recent afternoon from an overlook, at a safe distance from the blast. To the west was the San Rafael Valley, a rolling yellow grassland that is one of the last wild open spaces along the U.S.-Mexico border. Ringed by mountains, it has served as a setting for John Wayne Westerns and episodes of Little House on the Prairie. I saw no power lines, paved roads, or other signs of human presence, aside from the new camp where Trump’s workers were sleeping in trailers and crushing rocks to make concrete for the wall’s base. They had about 20 more miles to go to finish the whole valley, one of the last places in southeast Arizona that hasn’t been walled off.