<Tr> <Td> </Td> <Td> 1854 </Td> <Td> 76,845 km (29,670 sq mi) </Td> </Tr> <Tr> <Td_colspan="2"> Today part of </Td> <Td> Arizona and New Mexico in the United States of America </Td> </Tr> <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 that took effect on June 8, 1854 . The first draft was signed on December 30, 1853, by James Gadsden, U.S. ambassador 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 transmitted 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 . 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 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. needed to build a transcontinental railroad along a deep southern route, which the Southern Pacific Railroad later completed in 1881--1883 . The purchase also aimed to resolve border issues . </P>

Why did the us make the gadsden purchase