 Fearing that the <unk> could occur again , President Thomas Jefferson proposed to buy the port of New Orleans from France for $ 10 million . Instead , faced with a debt crisis , Napoleon offered to sell the entirety of Louisiana , including the Missouri River , for $ 15 million – amounting to less than 3 <unk> per acre . The deal was signed in 1803 , doubling the size of the United States with the acquisition of the Louisiana Territory . In 1803 , Jefferson instructed Meriwether Lewis to explore the Missouri and search for a water route to the Pacific Ocean . By then , it had been discovered that the Columbia River system , which drains into the Pacific , had a similar latitude as the headwaters of the Missouri River , and it was widely believed that a connection or short <unk> existed between the two . However , Spain <unk> at the takeover , citing that they had never formally returned Louisiana to the French . Spanish authorities warned Lewis not to take the journey and forbade him from seeing the MacKay and Evans map of the Missouri , although Lewis eventually managed to gain access to it . 
