On July 4, 1803, only 27 years after the fight for independence and when Thomas Jefferson was President of the United States, the country grew to twice its size as a result of the Louisiana Purchase. With the addition of the Louisiana territory's 530 million acres, the country saw a drastic change. For a total of $15 million, the United States obtained proprietorship of the land that extends east to west from the Mississippi River to the Rocky Mountains and north to south from the border with Canada all the way down to the Gulf of Mexico.

As it happens, Napoleon Bonaparte had just recently added the Louisiana territory to the list of French territorial possessions in North America. However, because war with the UK was looming, and France lacked funding for the war, Napoleon found himself offering the territory to the United States at the incredibly low price of 3 cents per acre. The Louisiana Purchase Treaty was signed on April 30, 1803 by the United States and France even with the opposition presented by Congress because there were doubts about the constitutionality of the transaction. In the end, Thomas Jefferson was able to exert the power of treaty negotiation granted to the presidency by the Constitution to complete the purchase.

In 1800 Napoleon acquired the territory from Spain to establish a French colony in North America. Prior to this acquisition, the Spanish crown had allowed American traders and farmers to use the Mississippi River to move their goods and to establish storage centers in the city of New Orleans. After Napoleon's acquisition of the territory, which was named after French King Louis XV, this permission was canceled, and French troops secured the city. Because of this and the insistence of many plantation owners in the South who feared that the French government would free the slaves leading to rebellion and conflict, the United States considered going to war with France to dispute ownership of the territory. To prevent the outbreak of another armed conflict, Thomas Jefferson consulted with a French friend and offered to purchase the territory instead of declaring a territorial war with France.

Although the Louisiana Purchase wasn't ratified by Congress until October 20, 1803, Thomas Jefferson, President of the United States, formally announced to the American people that the Purchase had been carried out and the documents transferring the land had been signed by the French government.