Immigration played a large role in westward expansion of the United States. Many of those who settled the West came to the United States from overseas in hopes of greater opportunity. For example, many immigrants from Scandinavia settled across the plains, and at the same time, there were also significant populations of Germans and Irish moving to the West. In addition to these immigrant groups, there was also a significant increase in immigration from Asia to the West Coast of the United States. Mexicans made up a significant portion of the Southwest, which was territory that had been won by the United States from Mexico in the Mexican-American War. Immigrants also played a big role in connecting the nation through the construction of the Transcontinental Railroad. Many immigrants worked low-paying jobs in poor conditions to accomplish construction of the railroad.

Although immigrants were key to settling the West, it is important to remember that they often faced hostility from other Americans. Groups like the Know-Nothing Party opposed immigration, primarily immigration from Catholic countries. In the West, Chinese immigrants were often discriminated against, including by the US government through the Chinese Exclusion Act.

The main importance of immigration for westward expansion is that immigration provided many of the farmers and laborers who made this expansion possible.  Let us look at two examples of this.  First, many of the men who worked on the transcontinental railroad were immigrants.  In the East, there were many Irish immigrants while many Chinese worked in the West.  Second, many of the people who came to the Great Plains to farm were immigrants from Germany and Scandinavia.  Thus, immigrants built the railroads that facilitated westward expansion and immigrants came to farm the lands gained during that expansion.