Louisiana was originally a Spanish territory. It had been claimed by Spain before anybody else owned it (other than the natives of the area). A treaty was made with France during the dictatorship of Napoleon that granted the area of Louisiana to France. The city of New Orleans sat on the border between territories of the United States and those of Louisiana, and so whenever trade boats were sent down the Mississippi river, they had to stop at the French port of New Orleans. The United States offered to buy the city of New Orleans from the Napoleon. Now, you would think Napoleon would never agree to this deal, but because there was a slave rebellion in Haiti (the island that the French got most of their war finances from), Napoleon was in need of funds to continue the Napoleonic Wars. Napoleon decided that it would be profitable to attempt to sell all of Louisiana to the United States. To this the President, Thomas Jefferson, agreed. The land was sold at a dirt cheap price of three cents per acre. Remember, the French had gained it for pretty much nothing, so any money was a profit, and Napoleon wanted to make sure the deal was accepted. All of the land, in total, cost about fifteen million United States Dollars (USD). If we adjust for inflation, then it becomes the even more astounding amount of three hundred and fourty two million USD (332 million USD).
Once the land was procured and the deal was done, Thomas Jefferson decided it would be best to send a team to explore the new territory. He chose his friend, Merriwether Lewis to led the expedition into the new territories. The goal of the expedition was to attempt to discover a river that lead to the Pacific Ocean. Lewis elected to bring his old friend from when they served together in the army. His name was William Clark. They also brought 31 other volunteers on their journey. In May of 1804, The expedition set out from Wood River, Illinois. The journey was made mostly by boat. They began by traveling up the Missouri river, through what would become the State of Missouri, and from there up the what would become the borders of Nebraska and Iowa, through what would become the Dakota states, and then west through Montana. Here they reached the Rocky Mountains, and had to cross over. They then followed some other rivers down to the Pacific Ocean. In 1805 they reached their destination. The return trip followed mostly the same path as the trip there, except that around the Rocky Mountains, Lewis and Clark split up so that Clark could explore some more territory. They rejoined their groups, and made their way back to St. Louis. They arrived in the year 1806. There was only one death, and it was an unpreventable health issue.
The Louisiana Purchase more than doubled the size of the United States. Fifteen new states would eventually be made up of this territory. Lewis and Clark explored the land looking for a new waterway, and ultimately, there was none, but they did succeed in establishing friendly relations with the Indians and in making maps of the new territory.