The Gadsden Purchase (known as Venta de La Mesilla, or Sale of La Mesilla, in Mexico) is a 29,640-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 on December 30, 1853 by James Gadsden who was the American ambassador to Mexico at that time. It was then ratified, with changes, by the U.S.