Gadsden Purchase, strip of land purchased (1853) by the United States from Mexico. The Treaty of Guadalupe Hidalgo (1848) had described the U.S.-Mexico boundary vaguely, and President Pierce wanted to insure U.S. possession of the Mesilla Valley near the Rio Grande—the most practicable route for a southern railroad to the Pacific. James Gadsden negotiated the purchase, and the U.S. Senate ratified (1854) it by a narrow margin. The area of c.30,000 sq mi (77,700 sq km), purchased for $10 million, now forms extreme S New Mexico and Arizona S of the Gila.