In late April 1803, with the stroke of a pen and the exchange of just $15 million, the United States nearly doubled in size. With the Louisiana Purchase, the U.S. acquired nearly 827,000 square miles of French-held land for just four cents an acre.

The purchase was about more than land alone—westward expansion, national wealth and international relations hung in the balance. But its human cost is still felt today.

Louisiana at the time covered most of the Mississippi Valley. Though people had lived there for thousands of years, it became the site of a fierce tussle over colonial power in the 18th century. France had once owned a massive swath of what is now the U.S.—including Louisiana. But after battling with