James Gadsden, U.S. Minister to Mexico, and General Antonio López de Santa Anna, President of Mexico, signed the Gadsden Purchase in Mexico City on December 30, 1853. The treaty settled the dispute over the exact location of the Mexican border west of El Paso, Texas, giving the U.S. claim to approximately 29,000 square miles of land in what is now southern New Mexico and Arizona, for the price of $10,000,000.
Source: The Library of Congress