The Gadsden Purchase, also known as the Treaty of Mesilla, is a 29,640-square-mile (76,800 km2) region of present-day southern Arizona and southwestern New Mexico that the United States acquired from Mexico. The first draft was signed on December 31, 1853, by James Gadsden, US Minister to Mexico, and by Antonio López de Santa Anna, president of Mexico. The treaty took effect on June 8, 1854.