In 1803, the U.S. sought to secure it's hold on the port of New Orleans in order to placate farmers in western states. France, the owner of New Orleans, needed money to support the Napoleonic wars and was motivated to sell more territory since Haiti won its independence. Napoleon offered the American diplomats most of western North America. Thomas Jefferson, the U.S. president in 1803, agreed to the purchase without the explicit consent of Congress. The United States bought over eight hundred thousand square miles of land for fifteen million dollars.

The significance of this move was huge. Jefferson, himself a naturalist, sent Lewis and Clark on a voyage of discovery down the Missouri River in order to learn about the flora, fauna, and people who lived in this new territory. This was the first contact U.S. citizens had with Native Americans in the region. The voyage would open the door to further exploration by mountain men who sought beaver pelts in the early-nineteenth century. These mountain men would go on to find trails to the Willamette Valley, thus starting one of the largest voluntary emigrations in world history. The borders of the new territory were somewhat vague—this would lead to negotiations with Britain over the ownership of the Oregon Territory as well as a war with Mexico in the 1840s, which expanded U.S. claims even further. The influx of new land from the Louisiana Purchase gave the U.S. a treasure trove of farmland as well as mineral wealth; however, the debate over whether this new land would be free or enforce slavery would shape the politics of the first half of the nineteenth century and ultimately lead to the Civil War.