You might think that the Mexican Cession would have been enough, but the United States wanted a little more.
In 1853, the American minister to Mexico, James Gadsden (1788-1858), convinced Mexico to sell to the United States the Mesilla Valley, including what is now Arizona and New Mexico.
The price tag for the 30,000 square miles was $10 million. The aim of the purchase: mineral wealth and a route for a railroad that would bypass the Rocky Mountains.
The Gadsden Purchase completed the story of American acquisition of Mexican land.