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250px|thumbnail|The US/Mexico Border in Nogales along the Southern border of the Gadsden Purchase]]__NOTOC__
The Gadsden Purchase, or Treaty, was an agreement between the United States and Mexico, finalized in 1854, in which the United States agreed to pay Mexico $10 million for a 29,670 square mile portion of Mexico that later became part of Arizona and New Mexico. Gadsden’s Purchase provided the land necessary for a southern transcontinental railroad and attempted to resolve conflicts that lingered after the Mexican-American War.
While the Treaty of Guadalupe Hidalgo formally ended the Mexican-American War in February 1848, tensions between the Governments of Mexico and the United States continued to simmer over the next six years. The two countries each claimed the Mesilla Valley as part of their own country.