In April 1803, The United States, under President Thomas Jefferson, purchased 828,000 square miles (2,144,510 square km) of land from France. This land acquisition is commonly known as the Louisiana Purchase.

The lands included in the Louisiana Purchase were those west of the Mississippi River but they were largely unexplored and therefore completely unknown to both the U.S. and France at the time. Because of this, shortly after the purchase of the land President Jefferson requested that Congress approve $2,500 for an exploratory expedition west.

Once Congress approved the funds for the expedition, President Jefferson chose Captain Meriwether Lewis as its leader. Lewis was chosen mainly because he already had some knowledge of the west and was an experienced Army officer. After making further arrangements for the expedition, Lewis decided he wanted a co-captain and selected another Army officer, William Clark.

The goals of this expedition, as outlined by President Jefferson, were to study the Native American tribes living in the area as well as the plants, animals, geology and terrain of the region. The expedition was also to be a diplomatic one and aid in transferring power over the lands and the people living on them from the French and Spanish to the United States. In addition, President Jefferson wanted the expedition to find a direct waterway to the West Coast and the Pacific Ocean so westward expansion and commerce would be easier to achieve in the coming years.

The Expedition
Lewis and Clark's expedition officially began on May 21, 1804 when they and the 33 other men making up the Corps of Discovery departed from their camp near St. Louis, Missouri. The first portion of the expedition followed the route of the Missouri River during which, they passed through places such as present-day Kansas City, Missouri and Omaha, Nebraska.

On August 20, 1804, the Corps experienced its first and only casualty when Sergeant Charles Floyd died of appendicitis. He was the first U.S. soldier to die west of the Mississippi River. Shortly after Floyd's death, the Corps reached the edge of the Great Plains and saw the area's many different species, most of which were new to them. They also met their first Sioux tribe, the Yankton Sioux, in a peaceful encounter.

The Corps next meeting with the Sioux however, was not as peaceful. In September 1804, the Corps met the Teton Sioux further west and during that encounter one of the chiefs demanded that the Corps give them a boat before being allowed to pass. When the Corps refused, the Tetons threatened violence and the Corps prepared to fight. Before serious hostilities began though, both sides retreated.

The Corps' expedition then successfully continued upriver until winter when they stopped in the villages of the Mandan tribe in December 1804. While waiting out the winter, Lewis and Clark had the Corps built Fort Mandan near present-day Washburn, North Dakota, where they stayed until April 1805. During this time, Lewis and Clark wrote their first report to President Jefferson. In it they chronicled 108 plant species and 68 mineral types. Upon leaving Fort Mandan, Lewis and Clark sent this report, along with some members of the expedition and a map of the U.S. drawn by Clark back to St. Louis.

Afterward, the Corps continued along the route of the Missouri River until they reached a fork in late May 1805 and were forced to divide the expedition to find the true Missouri River. Eventually, they found it and in June the expedition came together and crossed the river's headwaters. Shortly thereafter the Corps arrived at the Continental Divide and were forced to continue their journey on horseback at Lemhi Pass on the