In the 1853 Gadsden Purchase, what country sold land to the United States?

The Gadsden Purchase is a 29,670-square-mile (76,800 km2) region of present-day southern Arizona and southwestern New Mexico that the United States purchased via a treaty signed on December 30, 1853, by James Gadsden, U.S. ambassador to Mexico at that time. The U.S. Senate voted in favor of ratifying it with amendments on April 25, 1854, and then transmitted it to 14th President Franklin Pierce. Mexico's government and its General Congress or Congress of the Union took final approval action on June 8, 1854. The purchase was the last substantial territorial acquisition in the contiguous United States.

As the railroad age evolved, business-oriented Southerners saw that a railroad linking the South with the Pacific Coast would expand trade opportunities. They thought the topography of the southern portion of the original boundary line to the Mexican Cession (future states of California, Nevada, Utah, Arizona, New Mexico, western Colorado) of 1848 was too mountainous to allow a direct route. Projected southern railroad routes tended to veer to the north as they proceeded eastward, which would favor connections with northern railroads and ultimately favor northern seaports. Southerners saw that to avoid the mountains, a route with a southeastern terminus might need to swing south into what was still Mexican territory. The financially-strapped government of Antonio López de Santa Anna agreed to the sale, which netted Mexico $10 million dollars.