There are a few key events during this period that helped to open the West for the United States. Progress was helped by the advancement of the railroad industry and the discovery of several ore deposits in the region. Also, the Mexican-American War spurred territory development.
The first development that took place during this period was the development of routes to the West; specifically the Oregon, California, and Mormon Trails that allowed settlers to move West via wagon trains. Many of the routes had already been mapped by mountain men prior to 1820. Easterners looking for cheap land and hoping to escape financial collapses saved their money in order to join wagon trains. The most popular starting places, jumping off points, were towns such as Independence, Missouri and Council Bluffs, Iowa. The Oregon Trail ended in Oregon City. The California Trail sent prospective miners to the new goldfields around Sutter's Mill. Mormons took a wagon trail to Salt Lake City so they could practice their religion in peace. By 1850, these wagon trails were well-rutted and lined with trading posts and forts to offer settlers places to rest.
The next development was the Mexican-American War, which lasted from 1846-1848. This conflict settled the Texas border and led to the United States's annexation of much of the West. Many Americans viewed the territorial takeover as confirmation of Manifest Destiny. The new land would prove to be a curse, accelerating the conflict between slaveholders and abolitionists that ended in the Civil War.
The final development was the gold discovery at Sutter's Mill. This led to thousands of people from around the world coming to California via the Overland Trail or by ship. The discovery led to the destruction of Native Americans in Central California. The discovery also set the tone for future mineral discoveries in the West as miners created boom towns around strikes and quickly moved on when the land was no longer profitable.