U.S. President Donald Trump wants to make his campaign promise of building a wall on the Mexico-U.S. border come true. This would be a rather costly exercise, with unknown chances of succeeding in its primary goal of fending off unwanted immigrants to America. Mr. Trump has insisted that Mexico should carry the cost for the construction of the wall. Mexico's president, Enrique Pena Nieto, insists otherwise. About a third of the Mexico-U.S. border is already covered by various types of fences.