The Gadsden Purchase and a looming war

It was on this date in 1853 that the treaty finalizing the Gadsden Purchase was signed. Almost six years after the Treaty of Guadalupe Hidalgo concluded the Mexican War and established the border between Mexico and the United States, a cash-strapped Mexican government agreed to sell a strip of land that now makes up the southern border of Arizona and a small block of New Mexico's southern border with Arizona and Mexico.

The chief purpose of the acquisition was to accommodate a southern transcontinental railway, a move advocated by President Franklin Pierce's Secretary of War, Jefferson Davis. However, political events of the time doomed such a plan. 

The Kansas-Nebraska Act, which effectively replaced the Missouri Compromise of 1820, was sponsored by Illinois Senator Stephan Douglas. Douglas was a strong proponent of a central route for the nation's first transcontinental railroad, having successfully secured funding for a rail line that eventually linked Chicago to New Orleans. With the nation's attention fixed upon the controversial Kansas-Nebraska Act, support for federal monies to build a southern rail line evaporated and Jefferson Davis joined Douglas in support of both the new political compromise and expansion of a central transcontinental rail route.

https://constitutioncenter.org/blog/the-gadsden-purchase-and-a-failed-attempt-at-a-southern-railroad