---
title: "Thomas Jefferson"
chunk: 9/19
source: "https://en.wikipedia.org/wiki/Thomas_Jefferson"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T04:06:33.894887+00:00"
instance: "kb-cron"
---

Spain ceded ownership of the Louisiana territory in 1800 to France. Jefferson was concerned that Napoleon's interests in the vast territory would threaten the security of the continent and Mississippi River shipping. He wrote that the cession "works most sorely on the U.S. It completely reverses all the political relations of the U.S." In 1802, he instructed James Monroe and Robert R. Livingston to negotiate the purchase of New Orleans and adjacent coastal areas. In early 1803, Jefferson offered Napoleon nearly $10 million for 40,000 square miles (100,000 square kilometres) of tropical territory.
Napoleon realized that French military control was impractical over such a vast remote territory, and he was in dire need of funds for his wars on the home front. In early April 1803, he unexpectedly made negotiators a counter-offer to sell 827,987 square miles (2,144,480 square kilometres) of French territory for $15 million (~$380 million in 2024), doubling the size of the United States. U.S. negotiators accepted the offer and signed the treaty on April 30, 1803. Word of the unexpected purchase did not reach Jefferson until July 3, 1803. He unknowingly acquired the most fertile tract of land of its size on Earth, making the new country self-sufficient in food and other resources. The sale also significantly curtailed European presence in North America, removing obstacles to U.S. westward expansion.
Most thought that this was an exceptional opportunity, despite Republican reservations about the Constitutional authority of the federal government to acquire land. Jefferson initially thought that a Constitutional amendment was necessary to purchase and govern the new territory; but he later changed his mind, fearing that this would give cause to oppose the purchase, and urged a speedy debate and ratification. On October 20, 1803, the Senate ratified the purchase treaty by a vote of 24–7. Jefferson personally was humble about acquiring the Louisiana Territory, but he resented complainers who called the vast domain a "howling wilderness".
After the purchase, Jefferson preserved the region's Spanish legal code and instituted a gradual approach to integrating settlers into American democracy. He believed that a period of the federal rule would be necessary while Louisianans adjusted to their new nation. Historians have differed in their assessments regarding the constitutional implications of the sale, but they typically hail the Louisiana acquisition as a major accomplishment. Frederick Jackson Turner called the purchase the most formative event in American history.

=== Expeditions ===

Jefferson anticipated further westward settlements due to the Louisiana Purchase and arranged for the exploration and mapping of the uncharted territory. He sought to establish a U.S. claim ahead of competing European interests and to find the rumored Northwest Passage. Jefferson and others were influenced by exploration accounts of Le Page du Pratz in Louisiana (1763) and James Cook in the Pacific (1784), and they persuaded Congress in 1804 to fund an expedition to explore and map the newly acquired territory to the Pacific Ocean.
Jefferson appointed secretary Meriwether Lewis and acquaintance William Clark to lead the Corps of Discovery (1803–1806). In the months leading up to the expedition, Jefferson tutored Lewis in the sciences of mapping, botany, natural history, mineralogy, and astronomy and navigation, giving him unlimited access to his library at Monticello, which included the largest collection of books in the world on the subject of the geography and natural history of the North American continent, along with an impressive collection of maps. The expedition lasted from May 1804 to September 1806 and obtained a wealth of scientific and geographic knowledge, including knowledge of many Indian tribes.
Jefferson organized three other western expeditions: the William Dunbar and George Hunter Expedition on the Ouachita River (1804–1805), the Thomas Freeman and Peter Custis Expedition (1806) on the Red River, and the Zebulon Pike Expedition (1806–1807) into the Rocky Mountains and the Southwest. All three produced valuable information about the American frontier. This interest also motivated Jefferson to meet the Prussian explorer Alexander von Humboldt several times in June 1804, inquiring into Humboldt's knowledge of New Spain's natural resources, economic prospects, and demographic development.

=== Native American affairs ===