Even as late as 1840, it seemed to many that the Mississippi River would remain the boundary of the frontier for many years to come. At that time the land just west of the Mississippi was thought to be mostly desert and not good for farming. Because of this misconception, many people interested in moving west did not want to settle there. They wanted to keep going, attracted by the glowing reports of fertile lands beyond the desert—all the way to California and Oregon.

In 1845 only twenty thousand white Americans lived west of the Mississippi. Much of the activity in this region centered on the fur trade. During the 1830s most of the fertile farmland had been claimed in the Midwest. This left many people looking for other places to settle. In 1830 a company of fur traders—Smith, Jackson, and Sublette—moved...