The Gadsden Purchase (known as Venta de La Mesilla, or Sale of La Mesilla, in Mexico) is a 29,670-square-mile (76,800 km2) region of present-day southern Arizona and southwestern New Mexico that was purchased by the United States in a treaty signed by James Gadsden, the American ambassador to Mexico at the time, on December 30, 1853. It was then ratified, with changes, by the U.S. Senate on April 25, 1854 and signed by President Franklin Pierce, with final approval action taken by Mexico on June 8, 1854. The purchase was the last major territorial acquisition in the contiguous United States, adding an area the size of Scotland to the United States.
The purchase included lands south of the Gila River and west of the Rio Grande. The Gadsden Purchase was for the purpose of the US's construction of a transcontinental railroad along a deep southern route. It was also related to reconciliation of outstanding border issues following the Treaty of Guadalupe Hidalgo, which ended the Mexican–American War of 1846–48.
As the railroad age progressed, business-oriented Southerners saw that a railroad linking the South with the Pacific Coast would expand trade opportunities. They thought the topography of the southern portion of the Mexican Cession was too mountainous to allow a direct route. Projected southern routes tended to run to the north at their eastern ends, which would favor connections with northern railroads and ultimately favor northern seaports. Southerners saw that to avoid the mountains, a route with a southeastern terminus might need to swing south into what was then Mexican territory.
The administration of President Franklin Pierce, strongly influenced by Secretary of War Jefferson Davis, saw an opportunity to acquire land for the railroad, as well as to acquire significant other territory from northern Mexico. In the end, territory for the railroad was purchased for $10 million ($243,629,630 today), but Mexico balked at any large-scale sale of territory. In the United States, the debate over the treaty became involved in the sectional dispute over slavery, ending progress before the American Civil War in the planning or construction of a transcontinental railroad