The Gadsden Purchase (Spanish: Venta de La Mesilla "La Mesilla sale") is a 29,640-square-mile (76,800 km) region of present-day southern Arizona and southwestern New Mexico that the United States acquired from Mexico by the Treaty of Mesilla, which took effect on June 8, 1854. The purchase included lands south of the Gila River and west of the Rio Grande where the United States wanted the construction of what is now known as the Sunset Route, a transcontinental railroad, to be carried out, which the Southern Pacific Railroad later completed in 1881–1883. This allowed for the railroad's construction to be shorter, easier, and straighter. Without said purchase, the railroad's expansion would have taken longer and been more expensive. The purchase also aimed to resolve other border issues.
The first draft was signed on December 30, 1853, by James Gadsden, U.S. Minister to Mexico, and by Antonio López de Santa Anna, president of Mexico. The U.S. Senate voted in favor of ratifying it with amendments on April 25, 1854, and then sent it to President Franklin Pierce. Mexico's government and its General Congress or Congress of the Union took final approval action on June 8, 1854, when the treaty took effect. The purchase was the last substantial territorial acquisition in the contiguous United States, and defined the Mexico–United States border. The Arizona cities of Tucson, Yuma and Tombstone are on territory acquired by the U.S. in the Gadsden Purchase.