The Louisiana Purchase almost doubled the area of the United States; this new acquisition had an area of 828, 000 square miles (2, 155, 500 square kilometers). The Louisiana Territory was west of the Mississippi River to the Rocky Mountains, strectching from the Gulf of Mexico to Canada.
President Jefferson wanted to make sure that the US could use the Mississippi River for commerce, so he had the American ambassador to France, Robert Livingston, and a special envoy, James Monroe (who would later be president), negotiate to buy the port of New Orleans. This would allow the US access between the Gulf of Mexico and the Mississippi River. They were very surprised when the French offered to sell the entire Louisiana Territory for 15 million dollars. The purchase was completed on April 30, 1803.
Soon after (in 1804), Jefferson sent Lewis and Clark to map the newly-acquired western US territory (they returned in 1806 with maps, newly-discovered animals, and information about Indian tribes). Lewis and Clark traveled from Missouri west to Oregon and back again, mapping much of the area.