On January 18, 1803, Thomas Jefferson requests funding from Congress to finance the Lewis and Clark expedition.

On February 24, 1803, the Supreme Court, led by Chief Justice John Marshall, decides the landmark case of William Marbury v. James Madison, Secretary of State of the United States and confirms the legal principle of judicial review—the ability of the Supreme Court to limit Congressional power by declaring legislation unconstitutional—in the new nation.

In one of the great surprises in diplomatic history, French Foreign Minister Charles Maurice de Talleyrand makes an offer to sell all of Louisiana Territory to the United States.

On April 30, 1803, representatives of the United States and Napoleonic France complete negotiations for the Louisiana Purchase, a massive land sale that doubles the size of the young American republic. What was known as Louisiana Territory comprised most of modern-day United States between the Mississippi and the Rocky Mountains, with the exceptions of Texas, parts of New Mexico, and other pockets of land already controlled by the United States. A formal treaty for the Louisiana Purchase, antedated to April 30, was signed two days later.

On October 20, 1803, the U.S. Senate approves a treaty with France providing for the purchase of the territory of Louisiana, which would double the size of the United States.