On October 20, 1803, the Senate ratified the Louisiana Purchase Treaty by a vote of twenty-four to seven. The agreement, which provided for the purchase of the western half of the Mississippi River basin from France at a price of $15 million, or approximately four cents per acre, doubled the size of the country and paved the way for westward expansion beyond the Mississippi.

Spain had controlled Louisiana and the strategic port of New Orleans with a relatively free hand since 1762. However, Spain signed the Treaty of San Ildefonso in 1800 under pressure from Napoleon Bonaparte, a secret agreement retroceding the territory of Louisiana to France.

News of the agreement eventually reached the U.S. government. President Thomas Jefferson feared that if Louisiana came under French control, American settlers living in the Mississippi River Valley would lose free access to the port of New Orleans. On April 18, 1802, Jefferson wrote a letter to Robert Livingston, the U.S. minister to France, warning that, “There is on the globe one single spot, the possessor of which is our natural and habitual enemy. It is New Orleans…”

Napoleon, faced with a shortage of cash, a recent military defeat in Santo Domingo (present-day Haiti), and the threat of a war with Great Britain, decided to cut his losses and abandon his plans for an empire in the New World. In 1803, he offered to sell the entire territory of Louisiana to the United States for $15 million.

Robert Livingston and James Monroe, whom Jefferson had sent to Paris earlier that year, had only been authorized to spend up to $10 million to purchase New Orleans and West Florida. Although the proposal for the entire territory exceeded their official instructions, they agreed to the deal. The Louisiana Purchase Treaty was dated April 30 and formally signed on May 2, 1803.

The bounds of the territory, which were not clearly delineated in the treaty, were assumed to include all the land between the Mississippi River and the Rocky Mountains, at that time known as the Stony Mountains. Just twelve days after the signing of the treaty, frontiersmen Meriwether Lewis and William Clark, younger brother of Revolutionary War officer George Rogers Clark, set out on an expedition to explore the newly acquired territory.

The purchase of the Louisiana Territory and the Lewis and Clark expedition marked the beginning of a century of conquest. As explorers, speculators, adventurers, and settlers pushed the territorial boundaries of the United States westward toward the Pacific coast, the notion of America as a nation always pushing toward new frontiers took hold in art, literature, folklore, and the national psyche.