The Gadsden Purchase (1853):

The Gadsden Purchase added a strip of land in what is now southern Arizona and New Mexico, completing the contiguous continental expansion of the United States. The United States paid Mexico $10 million for the territory, which was sought in part as a route for a southern transcontinental railroad. The Treaty of Guadalupe Hidalgo, not the Gadsden Purchase, acquired the much larger Mexican Cession.

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