The US purchase of about 29,640 square miles of land in 1853 from Mexico. Today the area is the southern quarter of Arizona and a small strip of New Mexico. It is bounded on the west by the Colorado River, the east by the Rio Grande, and the north by the Gila River.

The Gadsden Purchase was necessary due to a misunderstanding in the 1848 Treaty of Guadalupe Hidalgo, which had defined the border between the United States and Mexico on an innacurate map. Ah, those crazy 19th Century Treaties!

The government of Mexico, under the leadership of Antonio Lopez de Santa Anna was in severe need of finances, which eased the path for the treaty. The U.S. Senate ratified the treaty on April 25th, 1854 after a heated debate on the adding of more slave territory to the US. The Senate reduced the size of the land slightly, and lowered the purchase to $10 million from its original $15 million price.

The treaty was not well received in Mexico, and helped lead to the downfall of Santa Anna.

In 1853, newly elected President Franklin Pierce started handing out offices as patronage for the people who had helped him get elected.  Of particular concern to abolitionists in the North was the appointment of Mississippi senator Jefferson Davis as Secretary of War.  One of the tasks Pierce assigned to Davis was the survey of possible routes for a transcontinental railroad.

It is hardly surprising that Davis's report specified a route that connected to the South as the best route.  As it turns out, this route ran through the Mesilla Valley, which was not within the area specified by the Treaty of Guadeloupe Hidalgo.  At least, the residents of Mesilla believed they were in Mexico. so, on the advice of Davis, Pierce appointed James Gadsden, president of the South Carolina Railroad Company, as his Ambassador to Mexico. Gadsden's principal task was to secure title to this land.

Although Davis managed to get Congress to charter the Southern Pacific railroad in 1857, Gadsden died in 1858 with no railroad built.

And then, the Civil War intervened and changed the fortunes of Jefferson Davis and other would-be southern railroad barons.  The "Big Four" Northern railroad interests acquired the Southern Pacific railroad from the Government in 1861.

The railroad was actually built through the Gadsden Purchase (after evicting farmers who had settled on the railroad's original route in the meantime) between 1878 and 1883. The second transcontinental railroad link occurred inside the Gadsden Purchase in 1881 when the Santa Fe railroad connected an extension from Las Cruces New Mexico to the Southern Pacific at Deming, New Mexico.

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