Pike Expedition: United States Army Lieutenant Zebulon Pike begins an expedition from Fort Bellefontaine near St. Louis, Missouri, to explore the west.

The Pike Expedition was a military party sent out by President Thomas Jefferson and authorized by the United States government to explore the south and west of the recent Louisiana Purchase. Roughly contemporaneous with the Lewis and Clark Expedition, it was led by United States Army Lieutenant Zebulon Pike, Jr. who was promoted to captain during the trip. It was the first official American effort to explore the western Great Plains and the Rocky Mountains in present-day Colorado. Pike contacted several Native American tribes during his travels and informed them that the U.S. now claimed their territory. The expedition documented the United States' discovery of Tava which was later renamed Pikes Peak in honor of Pike. After splitting up his men, Pike led the larger contingent to find the headwaters of the Red River. A smaller group returned safely to the U.S. Army fort in St. Louis, Missouri before winter set in.

The United States Army (USA) is the primary land service branch of the United States Department of Defense. It is designated as the Army of the United States in the United States Constitution. It operates under the authority, direction, and control of the United States secretary of defense. It is one of the six armed forces and one of the eight uniformed services of the United States. The Army is the most senior branch in order of precedence amongst the armed services. It has its roots in the Continental Army, formed on 14 June 1775 to fight against the British for independence during the American Revolutionary War (1775–1783). After the Revolutionary War, the Congress of the Confederation created the United States Army on 3 June 1784 to replace the disbanded Continental Army.

A lieutenant is a junior commissioned officer rank in the armed forces of many nations, as well as fire services, emergency medical services, security services and police forces.