By the mid-19th century, the United States had expansionist fever, fueled by a concept called Manifest Destiny, which was the idea that it was the destiny of the United States to own and occupy the land from the Atlantic to the Pacific. Forget Native Americans, Mexicans, or anyone else who happened to be living there and had lived there since forever. Americans were special and had a moral right to settle all that land to fulfill the promise of liberty and self-government. Some people even believed that this destiny was ordained by Providence, a.k.a. God.

Many areas to the west were acquired by a combination of battles, revolts, and treaties. (Think Florida, California, the Oregon Territory, and Texas.) Others were bought. In 1853, James Gadsden, the ambassador to Mexico, arranged a deal to buy 30,000 square miles in present-day southern Arizona and New Mexico for $10 million.

Like Napoleon at the time of the Louisiana Purchase, Mexican president Santa Anna (hero or villain of the Battle of the Alamo, depending on whose side you're on) needed the cash. The United States needed the land because it had big plans for a southern route for a transcontinental railroad to fuel its westward expansion. The deal didn't end tensions between Mexico and the United States, but it freed up the route for the railroad and set the present-day southern border of the United States.

The sale didn't go over too well in Mexico, and Santa Anna's political career was over.

Fun fact: Santa Anna was a huge fan of Napoleon and liked to imagine himself as "the Napoleon of the West." He emulated his battle strategies and read everything he could get his hands on about the French leader. Unfortunately, the similarities extended to several exiles, including the last one in—wait for it—Staten Island, New York (source).