Louisiana Purchase

The Louisiana Purchase was a land purchase made by President Thomas Jefferson in 1803. He bought the Louisiana territory from France, which was being led by Napoleon Bonaparte at the time, for 15,000,000 USD (about $320,000,000 in 2020 dollars). First, the men sent to France were allowed to spend up to 10 million USD in order to buy New Orleans and, if possible, the west bank of the Mississippi River. But then the French government said that for 5 million more dollars they would sell all of the Louisiana territory. Thomas Jefferson approved the deal and used his constitutional power to sign treaties to buy the land. Nowhere in the constitution does it say the president has the power to buy land from other countries, so he bought it with his power to make treaties. 
Napoleon Bonaparte sold the land because he needed money for the Great French War. The British had re-entered the war and France was losing the Haitian Revolution and could not defend Louisiana. Thomas Jefferson took the French offer as an opportunity to make America larger, even if it meant going against his Republican principles of small government (some would say he exceeded his constitutional power by accepting the deal on his own).
The purchase added 828,394 square miles and doubled the size of the U.S.. This included all of the states of Oklahoma, Nebraska, Kansas, Iowa, Missouri, and part of the states of Louisiana, Texas, Minnesota, Colorado, New Mexico, Wyoming, Montana, North Dakota, and South Dakota. It also included a small piece of land that is now part of Canada.
The Louisiana Purchase gave the U.S. control of the Mississippi River and the port city of New Orleans, both of which were used by farmers to ship their crops and get paid. It also ensured that France and other European countries would not try to take the land. France only controlled small bits of the territory. Because of that, the United States had to make other agreements with and payments to other governments and groups. The total amount paid for the Louisiana Purchase and all of these other agreements was about 2.6 billion USD.
The Lewis and Clark expedition explored the Louisiana Purchase and the Oregon Territory. They started from St. Louis. Their route traced the Missouri River.