The Gadsden Purchase is a 29,640-square-mile (76,800 km2) region of present-day southern Arizona and south western New Mexico that was purchased by the United States in a treaty signed on 30 December, 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 25 April, 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 8 June, 1854. The purchase was the last territorial acquisition in the contiguous United States to add a large area to the country.
The purchase included lands south of the Gila River and west of the Rio Grande; it was largely so that the U.S.A. could construct a transcontinental railroad along a deep southern route. (This happened with the transcontinental railroad, constructed by the Southern Pacific Railroad in 1881/1883). It also aimed to reconcile outstanding border issues between the U.S. and Mexico following the Treaty of Guadalupe Hidalgo, which ended the earlier first Mexican–American War of 1846–1848.