The Gadsden Purchase is a 29,640 mi² (77,700 km²) region of what is today southern Arizona and New Mexico that was purchased by the United States from Mexico in 1853. The purchase included lands south of the Gila River and west of the Rio Grande.

The purpose of the purchase was to allow for the construction of a southern route for a transcontinental railroad. Another rationale for the purchase was to give Mexico more money in compensation for the small amount paid for the lands taken by the United States five years earlier in 1848 in the Treaty of Guadalupe Hidalgo that ended the Mexican-American War. On December 30, 1853, U.S. Minister to Mexico James Gadsden and Mexican President Antonio López de Santa Anna agreed on the price of (USD) $10 million for the land. That's about $337.38 per mi².

The original plans of the purchase called for a much larger portion of land to be acquired from Mexico extending far enough south to encompass most of the current Mexican states of Coahuila, Chihuahua, and Sonora as well as all of the Baja California peninsula. These original boundaries were not only opposed by the Mexican people, but also by anti-slavery Senators who saw this as a move towards the acquisition of more slave territory. Even the small final strip of land that was finally acquired was enough to anger the Mexican people who saw Santa Anna's actions as yet another betrayal of their country and watched in dismay as he squandered the money. This would be one of the major contributing factors that led to the end of Santa Anna's political career.

The lands of the purchase were added to the existing New Mexico Territory. In order to help control the new land, the United States Army established Fort Buchanan on Sonoita Creek in present-day southern Arizona on November 17, 1856. Nevertheless, the difficulty of governing the new areas from the territorial capital at Santa Fe led to efforts as early as 1856 to organize a new territory out of the southern portion of the New Mexico Territory. Many of the early settlers in the region were, however, pro-slavery and sympathetic to the South, resulting in an impasse in Congress over how to reorganize the territory.

In 1861, after the start of the American Civil War, the Confederacy formed the Confederate Territory of Arizona largely out of the areas of the purchase. In 1863, using a north-to-south dividing line, the Union created its own Arizona Territory out of the western half of the New Mexico Territory, including most of the lands acquired in the purchase.

See also

• Mexican Cession
• Historic regions of the United States