Gadsden Purchase, a tract of land bought by the United States from Mexico in 1853 for $10,000,000. The United States gained 29,640 square miles (76,767 km2) of territory in what is now Arizona and New Mexico. The sale extended the United States-Mexico boundary southward between the Rio Grande on the east and the Colorado River on the west.

President Franklin Pierce wanted the land because it offered a practical route for a southern transcontinental railway. The purchase was negotiated between James Gadsden (1788–1858), minister to Mexico, and the Mexican dictator, Santa Anna. It was ratified by the U.S. Senate in June, 1854. Public opinion in Mexico was bitterly opposed to the sale, and it was one of the reasons for Santa Anna's downfall in 1855.