What was the smallest expansion of the US border and when did it take place?

The smallest expansion of the US border occurred in 1853 with the Gadsden Purchase, where the United States acquired approximately 29,670 square miles of land from Mexico for $10 million. This area, located in present-day southern Arizona and New Mexico, was intended to facilitate the construction of a southern transcontinental railroad. The purchase is notable for its relatively modest size compared to other territorial expansions in US history.