Mexico sold the Gadsden Purchase territory to the United States in 1853. The agreement transferred about 29,670 square miles of land in what is now southern Arizona and southwestern New Mexico.
The purchase was negotiated by U.S. minister James Gadsden and Mexican President Antonio López de Santa Anna’s government. The United States paid Mexico $10 million for the territory. American officials wanted the land partly because its southern route could support a potential transcontinental railroad.
The Treaty of La Mesilla was signed on December 30, 1853, and the transfer was completed in 1854 after the United States and Mexico adjusted the original plan. The purchase is sometimes confused with the much larger Mexican Cession, which followed the Mexican-American War and was acquired under the Treaty of Guadalupe Hidalgo in 1848. The Gadsden Purchase was the last major territorial acquisition in the contiguous United States.