In the Louisiana Purchase of 1803, the United States received land from France making up eastern and Central Colorado. U.S. exploration of the area began soon thereafter. Zebulon Montgomery Pike led a party into Colorado in 1806. The area's semi-permanent white inhabitants were the so-called "mountain men," who were trappers and fur traders. The remainder of what became Colorado passed from Spain to Mexico with the latter's independence in 1821.

The Treaty of Guadalupe Hidalgo, which ended the Mexican War in 1848, gave the area to the United States. Discovery of gold near what now is Denver caused an influx of settlers. Mining camps sprung into existence. Ignoring Indian claims, settlers proclaimed the Territory of Jefferson, which was not recognized by the U.S. Congress. The Colorado Territory was organized in 1861. Colorado was admitted to the Union in 1876. The Indian wars, which had continued through the 1860s and 1870s, came to an end in 1880 when a treaty was signed with the Ute Chief Ouray. In 1881 the Indians were deported to reservations.