Louisiana Purchase completed

On April 30, 1803, American envoys Robert R. Livingston and James Monroe signed the Louisiana Purchase treaty in Paris with representatives of Napoleon Bonaparte. The agreement transferred roughly 828,000 square miles of French territory west of the Mississippi River to the United States for $15 million. President Thomas Jefferson accepted the purchase even though he had long argued for a narrow reading of federal power under the Constitution.

The purchase intensified a constitutional and political tension at the center of Jefferson's presidency: strict construction gave no explicit guidance for acquiring vast foreign territory by treaty, yet control of New Orleans and the Mississippi River was economically vital to western Americans. Jefferson chose practical statecraft over prior constitutional scruple because a French empire on the Mississippi threatened American commerce and security. The treaty therefore showed that national expansion could force even committed Republicans to stretch their own theory of federal power.

The Senate ratified the treaty in October 1803, and the acquisition doubled the size of the United States almost overnight. The purchase also led directly to the territorial administration of Louisiana and to the Lewis and Clark Expedition, which Jefferson used to explore and map the new acquisition.