<P> The Pike Expedition (July 15, 1806--July 1, 1807) 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 of the new US rule over the territory . The expedition documented the United States' discovery of Pikes Peak . 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 US Army fort in St. Louis, Missouri before winter set in . </P> <P> Pike's company made several errors and ended up in Spanish territory in present - day Southern Colorado, where the Americans built a fort to survive the winter . Captured by the Spanish and taken into Mexico in February, their travels through present - day New Mexico, Mexico, and Texas provided Pike with important data about Spanish military strength and civilian populations . Although he and most of his men were released because the nations were not at war, some of his soldiers were held in Mexican prisons for years, despite US objections . In 1810, Pike published an account of his expeditions, which was so popular that it was translated into French, German, and Dutch for publication in Europe . </P>

Who explored the southwestern portion of the louisiana purchase