The Gadsden Purchase

Six years after the Mexican-American War redrew the border, a strip of desert in present-day southern Arizona and New Mexico still stood between the United States and its ambitions. In 1854 the United States bought it from Mexico for $10 million in a deal negotiated by James Gadsden — the last piece of territory added to the contiguous United States.

The motive ran on rails. Surveyors had concluded that the best southern route for a transcontinental railroad ran just below the existing border, through land still held by Mexico. Rather than reroute the dream of a southern railroad, the United States simply bought the ground it would run on.

For Mexico, still reeling from the loss of half its territory in 1848, the sale was bitterly unpopular and helped topple the government of Antonio López de Santa Anna. For the United States, it was a quiet capstone to a generation of expansion — the moment the continental map of the lower 48 was effectively complete.

The purchase also sharpened the sectional crisis at home. A southern railroad route was a southern prize, and the wrangling over it fed directly into the Kansas-Nebraska Act the same year. The last addition to the map was, like so much in the 1850s, entangled with the coming fight over slavery.