What did the Louisiana Purchase accomplish?

The Louisiana Purchase accomplished the most consequential territorial expansion in American history by giving the United States control of the entire Mississippi River, the port of New Orleans, the Missouri River system, and a vast interior reaching to the Rocky Mountains. The most immediate accomplishment was control of the Mississippi River and the port of New Orleans. Before 1803, American farmers in Kentucky, Tennessee, and the Ohio Valley shipped their crops down the river system to New Orleans, where they were transferred to oceangoing vessels for sale on the East Coast or in Europe. Spain held New Orleans until October 1, 1800, then France until December 20, 1803, and either power could choke American commerce by closing the port or imposing heavy duties. Spanish Intendant Juan Ventura Morales had revoked the right of deposit at New Orleans on October 16, 1802, panicking western farmers. Permanent American ownership of the port secured the western economy and prevented endless future crises.

The second accomplishment was doubling the territory of the United States. The country grew from about 870,000 square miles to roughly 1.7 million square miles, providing land for generations of farmers and the foundation of westward expansion. The Louisiana territory contains all or part of 15 modern states including Arkansas, Iowa, Kansas, Louisiana, Minnesota, Missouri, Montana, Nebraska, North Dakota, Oklahoma, South Dakota, Wyoming, and pieces of Colorado, New Mexico, and Texas. Some of the most fertile farmland in the world, including the prairies that became the breadbasket of the country, was now American.

The third accomplishment was geopolitical. By acquiring French claims, the United States eliminated a great power neighbor on its western flank and pushed back European influence in North America. The next century of American foreign policy in the hemisphere, including the Monroe Doctrine of December 2, 1823, would have been impossible without the strategic depth the purchase created.

The fourth accomplishment was constitutional. Although Thomas Jefferson worried that the Constitution did not authorize territorial acquisition, the precedent set by the Louisiana Purchase established that the federal government could acquire and govern new territory and admit new states from it on equal footing, a process formalized in the Northwest Ordinance of 1787 and now extended to the trans-Mississippi West.

The fifth accomplishment was scientific and ethnographic. The Lewis and Clark Expedition from May 1804 to September 1806 surveyed the territory, catalogued plants and animals, and made first formal contact with dozens of Native nations including the Mandan, Hidatsa, Shoshone, and Nez Perce. The Louisiana Purchase set in motion the entire trajectory of nineteenth century American history.