On December 5, 1800, Meriwether Lewis was promoted to captain in the 1st Infantry. Lewis joined the Army in 1795 as an Ensign (equivalent to second lieutenant), having previously served in the Virginia militia for two years. One year later in 1801, Lewis would leave the Army and serve as President Thomas Jefferson’s personal aide in the White House. Jefferson personally contacted the commanding general of the Army and asked that Lewis be allowed to leave the service, while retaining his rank and right to promotion. Lewis personally acted as Jefferson’s military advisor, as Jefferson had no military experience.
This close proximity to President Jefferson would lead to Lewis leading one of the great explorations in American history. Jefferson completed the Louisiana Purchase in July 1803 and immediately set into motion a crash course for Lewis. Lewis studied under the leading scientists of the time, so that he could properly report his findings. The Corps of Discovery departed from Missouri in May 1804 on a two-year mission of exploration. The team would return to Missouri in September 1806.
On August 17, 1803, members of the 1st infantry, under Captain John Whistler, established Fort Dearborn. Fort Dearborn was the first permanent military presence in Chicago, IL. In March 1803, the Secretary of War instructed Colonel Hamtramck, commander of the 1st Infantry and commandant of Detroit, to dispatch one officer and six men to survey Chicago. Interest had increased in Chicago, since native tribes withdrew as a result of the 1795 Treaty of Greenville. The positive findings of the survey resulted in construction of Fort Dearborn. The fort, named for the Secretary of War who commissioned it, took one year to complete. Whistler was named commandant of Chicago and remained there until 1810.