The Gadsden Purchase involved the United States acquiring a tract of land from Mexico that encompassed parts of present-day southern Arizona and New Mexico. Negotiated under the aegis of James Gadsden, the United States sought this land to enable the construction of a transcontinental railroad along what was perceived to be a more favorable southern route. The purchase was motivated by both economic and strategic reasons, as the burgeoning United States was experiencing growth and needed to enhance its transport infrastructure in the aftermath of the Mexican-American War.

James Gadsden, leveraging his position as the U.S. Minister to Mexico, conducted negotiations with the Mexican government, led by President Antonio López de Santa Anna. The negotiations culminated in the signing of the treaty on December 30, 1853. The U.S. agreed to pay $10 million for an area that was primarily desert but deemed important for its railroad potential. This purchase altered the configuration of the southwestern border, further delineating U.S.-Mexican territory.

The acquisition also had implications for the burgeoning tensions surrounding the expansion of slavery in the newly acquired territories. Supporting the southern route of the railroad was seen as a way to consolidate slave state interests in the growing nation. While it succeeded in its immediate goals related to transportation, the Gadsden Purchase also contributed to the complex interplay of sectional tensions that would later erupt in the Civil War.