In 1801, Thomas Jefferson was elected as the third president of the United States, after George Washington and John Adams. In 1803, Jefferson acquired land from France shortly after their revolution had ended in what was called the Louisiana Purchase. Jefferson authorized the Captain Meriwether Lewis and Second Lieutenant William Clark expedition (aka the Lewis and Clark Expedition), which included Lewis and Clark as well as a select group of US Army volunteers who traveled through the newly acquired land from May 1804 to September 1806 to map out and explore the Louisiana Territory, as well as find a practical route across the territory to the Pacific Ocean, and to establish an American presence in the area before other European powers try to lay claim to it. Lewis and Clark were also aided in the expedition by a Shoshone Indian woman Sacagawea and her husband Toussaint Charbonneau. As a result of the Louisiana Purchase, the territory belonging to the US was doubled; and the Louisiana Purchase was the largest territorial gain in US history as the territory stretched from the Mississippi River to the Rocky Mountains (this territory would later be transformed to 15 states).
How did the US pay for the land? Jefferson paid $3 million in gold as a down payment and issued bonds to pay for the remainder to the French. The French, who were ruled by Napoleon Bonaparte at the time, needed the money as quickly as possible in order to finance the planned invasion of England which never took place. Through aiding the French, the US gained an expansive amount of land which would be at the forefront of history for years to come, especially when it came to the expansion of slavery into the western territories (the Trans-Atlantic slave trade had been deemed illegal in 1808 under Jefferson but the domestic slave trade still thrived and would continue to thrive until the presidency of Abraham Lincoln) as well as with the Indian Removal/Trail of Tears.