Gadsden Purchase, a tract of land acquired from Mexico by purchase. The treaty was negotiated by James Gadsden. The territory acquired lies within the present boundaries of Arizona and New Mexico. It is about 120 miles wide and extends along the southern bank of the Gila River from the Colorado to the Rio Grande, comprising in all about 45,535 square miles. The treaty was negotiated in 1853. It was ratified in 1854. The United States government paid Mexico $10,000,000. The Southern Pacific Railway traverses the tract from east to west. The sale was one of the causes that led to the downfall of Santa Anna.