Also known as the Treaty of La Mesilla, the Gadsden Purchase was completed on December 30, 1853, by James Gadsden, US minister to Mexico. This transaction was the last addition of land made to the mainland United States. In 1844, President James K. Polk was elected president, and he was a strong believer in "Manifest destiny". This belief led to the annexation of Texas in 1845 and eventually a full-out war on Mexico which led to America conquering Mexico City.

     Despite the fact that many advisors to Polk were calling for the full annexation of Mexico, Polk signed the Treaty of Guadalupe-Hidalgo in 1848. This gave the United States much of the West including California, New Mexico, Arizona, Utah, Colorado, and Nevada, in exchange for $15 million. This addition of new territory led to the need for a transcontinental railroad to run from California to Texas.  However, during the construction of the railroad, the route planned ran through some mountainous regions, thus a new route was planned to run through the Gila River region. Thus in order for the railroad to run through this region the border between the United States and Mexico would have to be extended, thus came about the Gadsden Purchase.