Boundary between the United States and Mexico, map by William H. Emory, 1855
Between 1849 and 1857, the Army Corps of Topographical Engineers marked and mapped the U.S.-Mexico boundary and published a report on natural features of the borderland. The vast new western territories immediately became a focus of congressional dispute regarding slavery in new states. This map shows the border between New Mexico and Chihuahua, Mexico.
Geography and Map Division, Library of Congress