El Camino Real de Tierra Adentro, the Royal Road of the Interior, is the earliest Euro-American trade route of cultures and commerce in the United States. It spanned about 1,800 miles from Mexico City, where the road originated, to Santa Fe, in New Mexico. For three centuries, this Spanish colonial road followed a network of ancient Native American footpaths and trails that followed the wide expanse of the Rio Grande valley. There were parajes, or...