The treaty that ended the Mexican War in 1848 left some doubt about the western part of the boundary between Mexico and the United States. To clear up this question, and also to provide the United States with a good southern railroad route to the Pacific Coast, the government bought from Mexico a strip of land that included the region south of the Gila River in what is now Arizona and New Mexico. James Gadsden, the U.S. minister to Mexico, conducted the negotiations with Antonio Lopez de Santa Anna, the Mexican president. The treaty of sale was signed on Dec. 30, 1853, and the two countries exchanged ratifications of the treaty on June 30, 1854.

The United States paid $10 million for the 29,640 square miles in the purchase. Opposition in Mexico to the sale was one of the reasons for Santa Anna's banishment in 1855.