The event leading to the Gadsden Purchase becoming part of the U.S. was the negotiation and signing of the Gadsden Treaty. In 1853, the United States sought to acquire land for the construction of a southern transcontinental railroad route. The U.S. government sent James Gadsden as its negotiator to Mexico, offering to purchase a strip of land that is now southern Arizona and southern New Mexico. The Gadsden Treaty was signed on December 30, 1853, and ratified by both the U.S. Senate and the Mexican government in 1854. Thus, the Gadsden Purchase was formally incorporated into the United States.
