A small purchase made from Mexico to make a railroad to CA
What was it?
A very small territorial purchase made in 1853 in what is now southern New Mexico and Arizona. The US paid $10 million for a tiny sliver of land, 2/3rds of what they paid after the Mexican-American War for land that was literally 50x the size.
What did it do?
The US wanted this land so that a railroad could be constructed to California, south of the Rockies.
The US purchased this land for one reason: railroad. The money paid for it shows how important railroads were to the country in the 1850's. This was the last major territorial acquisition made by antebellum America.