The Louisiana Purchase was a pivotal moment in the expansion of the United States. Negotiated in 1803, it involved the acquisition of approximately 828,000 square miles of territory from France for $15 million, a sum that would amount to about four cents per acre. The region covered land that would eventually become all or parts of 15 states, including Arkansas, Missouri, Iowa, North Dakota, South Dakota, Nebraska, and Kansas. The deal was prompted by the desire of the United States to secure land for agriculture and settlement, as well as to ensure control of the Mississippi River, a crucial trade route for Western settlements.

President Thomas Jefferson was a principal architect of the deal, motivated by his belief that a nation of independent farmers was the foundation of a republic. He was particularly concerned that France, under Napoleon Bonaparte, might seek to expand its influence in North America, which could potentially threaten American expansionist aspirations. Diplomatically, the negotiations were coordinated by Robert Livingston and James Monroe, who were dispatched to France to negotiate the purchase of New Orleans and its surrounding areas. However, to their surprise, the French government offered the much larger territory.

The agreement was signed on April 30, with formal ratification following later in the year, despite concerns that it overstepped the constitutional power of the presidency. This acquisition not only doubled the geographical size of the United States but also set the stage for further explorations, including the famous Lewis and Clark Expedition, which was authorized by Jefferson to explore the newly acquired lands and find a route to the Pacific Ocean.