(Dec. 30, 1853), transaction that followed the conquest of much of northern Mexico by the United States in 1848. Known in Mexican history as the sale of the Mesilla Valley, it assigned to the United States nearly 30,000 additional square miles (78,000 square km) of northern Mexican territory (La Mesilla), now southern Arizona and southern New Mexico, in exchange for $10,000,000. Prompted in part by advocates of a southern transcontinental railroad, for which the most practical route would pass through the acquired territory, the purchase was negotiated by the U.S. minister to Mexico, James Gadsden (Gadsden, James). Residents of the territory were to enjoy the same protections as those afforded to residents of the area ceded to the United States earlier by the Treaty of Guadalupe Hidalgo (1848).