El Camino Real de Tierra Adentro, the Royal Road of the Interior, was a 1,600-mile braid of trails that led from Mexico City, in the center of New Spain, to the provincial capital of New Mexico on the edge of the empire's northern frontier. The Royal Road served as a lifeline for the colonial system from its founding in 1598 until the last days of Spanish rule in the 1810s. Throughout the Mexican and American Territorial periods, the Camino Real expanded, becoming part of a larger continental and international transportation system and, until the trail was replaced by railroads in the late...