At Coronado National Memorial in Arizona, the demolition crews blowing up national-park land tend to announce explosions at least a day in advance, as a warning for hikers to stay away. The crews have been working their way up the western slope of the park for the past couple of months, right along the international boundary with Mexico. President Trump's border wall needs a smooth, straight path, and there are mountains in the way. Trump didn't build along this stretch of the border during his first term, but his crews are now working at a furious pace. They have already completed about five miles of 30-foot-tall barrier, painted jet black at the president's insistence because he thought it looked more intimidating and would be hotter to the touch. 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...
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