The Louisiana Purchase doubled the size of the United States, adding over 800,000 square miles of territory west of the Mississippi River for $15 million. Facing financial troubles and the need to focus on war in Europe, Napoleon agreed to sell the Louisiana territory to the United States, easing President Thomas Jefferson's concerns about French domination in North America and access to important ports. The expedition of Meriwether Lewis and William Clark was commissioned to explore the new lands and report back to Jefferson.