The Gadsden Purchase (known in Mexico as , "Sale of La Mesilla") is a 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. Senate on April 25, 1854, and signed by 14th President Franklin Pierce, with final approval action taken by Mexico's government and their General Congress or Congress of the Union on June 8, 1854. The purchase was the last territorial acquisition in the contiguous United States to add a large area to the country.