<P> The Gadsden Purchase (known in Mexico as Spanish: Venta de La Mesilla, "Sale of La Mesilla") is a 29,670 - square - mile (76,800 km) region of present - day southern Arizona and southwestern New Mexico that the United States purchased via a treaty signed on December 30, 1853, by James Gadsden, U.S. ambassador to Mexico at that time . The U.S. Senate voted in favor of ratifying it with amendments on April 25, 1854, and then transmitted it to 14th President Franklin Pierce . Mexico's government and its General Congress or Congress of the Union took final approval action on June 8, 1854 . The purchase was the last substantial territorial acquisition in the contiguous United States . The U.S. sought the land as a better route for the construction of the southern transcontinental railway line, and the financially - strapped government of Antonio López de Santa Anna agreed to the sale, which netted Mexico $10 million (equivalent to $270 million in 2017). After the devastating loss of Mexican territory to the U.S. in the Mexican - American War (1846--48) and the continued filibustering by U.S. citizens, Santa Anna may have calculated it was better to yield territory by treaty and receive payment rather than have the territory simply seized by the U.S. </P> <P> The purchase included lands south of the Gila River and west of the Rio Grande which the U.S. acquired so that it could construct a transcontinental railroad along a deep southern route, which the Southern Pacific Railroad later completed in 1881 / 1883 . The purchase also aimed to reconcile outstanding border issues between the U.S. and Mexico following the Treaty of Guadalupe Hidalgo, which ended the earlier Mexican--American War of 1846--1848 . </P>

Why had the united states bought the land in the gadsden purchase