chapters in many mining towns. While this was not uncommon in Gilded Age America, the Wobblies were deemed radical by most other labor groups.

Economically, the West was a boon to the United States. Texas cattle brought to rail heads in Kansas and Nebraska helped to feed a growing Eastern population. The mineral wealth from the West helped to fund the Civil War. California's ports gave the United States greater access to Pacific markets. Valuable farmland in the Willamette Valley and California helped to bring Americans fruit year-round once the railroads adopted refrigerated cars. The climate in the West would also help to grow the movie industry at the beginning of the twentieth century.

This question is a bit ambiguous. The two answers so far have assumed that the question was about US expansion in America, but the question can also refer to Western expansion in general. If this is the case, then we are talking about colonialism, when the western powers of Europe were in a frenzy trying to take a piece of the rest of the world. If this is the case, then we can say that the West has it fingerprints in almost every sector of the world and they have developed it in numerous ways. This expansion has enriched the west in great ways and at times even brought wealth to other countries, but it has also brought bitterness, because some have felt exploited. The debates still go on.

The above answer very nicely lays out some effects of this expansion.  They are what I would call more philosophical effects -- things like its impact on the national discourse.  I would like to point to a few more mundane impacts:

• The expansion provided economic opportunities for people in the East.  It was the last chance to "go west," as the previous answer mentions, and try to improve your lot in that way.
• Relatedly, the opening of the West (and advertising about that in Europe) helped to attract a flood of immigrants (largely Scandinavian and German) who came specifically to move West and farm.  The German side of my family came in this way.
• It reduced food prices for people in the East.  The Western lands (when connected up by railroads to the East) could more produce food (especially things like grain and meat) much more efficiently and cheaply than Eastern farmers generally could.

There were several seismic changes that resulted from Westward Expansion.  One major change was the Native Americans were relegated to merely the fringes, literally and figurative, of the national discourse.  The prospecting for both precious metals and dollars caused Native Americans to be completely displaced in the process.  This began to impact national character as the predominant belief of the cultural and economic majority began to take form in the United States.  Industrialists were also able to maximize their economic powers with Westward Expansion.  Railroads, such as the Union Pacific and Transcontinental, were built with the hands and lives of millions of immigrants and low wage earners who had no recourse as their bosses profited in amazing quantities.  At the same time, the railroads' economic profit helped to develop more towns along the way and some of the railroads were actually extended to help develop economic growth in towns that ended up currying favor with industrialists.  The notion of "Go West, Young Man," and "Manifest Destiny" had converged in the development of economic and social frontiers, representing both the very best in American growth and worst in it is tendency to silence discourse.

I will expand on the previous educator's points.

Indeed, westward expansion increased the nation's wealth. Shortly after the Civil War, in 1865, only about 40 miles of track had been laid for the Union Pacific Railroad. The line was completed in 1869. The Union Pacific made it possible to travel from New York to San Francisco in ten days—a time frame that shortened with the development of higher-speed trains. The ability to go from one part of the country to another made it possible to transport goods and people more efficiently. The train eliminated the previous dangers that existed for those who traveled West on wagon trains, including attacks by Native tribes and harsh weather conditions.

Westward expansion became a greater possibility, not only because of the belief in Manifest Destiny (that it was the fate of the United States to stretch its borders from one ocean to another) but also due to the Monroe Doctrine. President James Monroe introduced a policy in 1823, in the midst of a presidential term, stating that the United States would stay out of European affairs as long as Europe stayed out of American affairs. This included two very important points: that Europe would not seek any colonies in the American West and that any attempt by Europe to control a nation in the Western hemisphere would be viewed by the US as an act of hostility. This doctrine ensured and validated American hegemony in the West.

The nation's ability to move westward also inspired imperial adventures elsewhere by the end of the nineteenth century. The US would later seize colonies in the Pacific, including Hawaii, and in the Caribbean, including Puerto Rico. Cuba had three years of military rule, starting in 1898, after the Spanish-American War.

To the previous educator's last point, westward expansion did create the pioneer culture that has been depicted in novels by Willa Cather and in films and television shows. Indeed, one had to be plucky and courageous to survive on the frontier, which was dangerous, inclimate, and sparsely populated. However, the repeated displacement and murder of indigenous people, who were removed in favor of homesteaders, calls into question the notion that westward expansion helped to create a more democratic nation. Furthermore, slavery did expand into some western territories, including Arizona Territory. Lastly, freed slaves had fewer opportunities than whites to acquire land out West.

Westward expansion impacted American society in many ways.  Here are three:

• It made American society richer.  Getting access to more land and more resources helped to make the country richer.
• It helped to promote the idea that the US had a "manifest destiny" to be a great power.  Expansion was based on taking land from people (Indians and Mexicans) who were deemed inferior.  This helped promote the idea that Americans were better than others and deserved more power.
• It is said to have helped create a more democratic and individualistic culture.  By going out west, people became more self-reliant because they had to face hardships on their own.  

Economically, westward expansion affected the United States mainly by opening up vast new amounts of natural resources.  For example, westward expansion opened the Great Plains to farming.  This allowed large amounts of grain to be grown on land that was really well-suited to the purpose, which meant that relatively poor lands in the East did not need to be used anymore.  It gave the US access to sources of metals, including famous strikes of gold in California and silver in the Comstock Lode in Nevada.  All of these resources and more were added to the US economy when the country expanded westward.

Politically, westward expansion exacerbated divisions between the North and the South.  This was particularly true in the time after the Mexican-American War.  When the US got the lands of the Mexican Cession, the North and South came into conflict over whether those lands would be free or slave.  The arguments between advocates of “free soil” and those who wanted to spread slavery into the new lands made the North and South even angrier at one another.  This helped to bring on the Civil War.

Socially, it is said that westward expansion helped to create the American character.  We see ourselves as a land of rugged individualists, the heirs to the pioneers who “tamed” the West.  Without westward expansion, we would not have our stories about cowboys and pioneers and other tough people who we see as archetypes of what we are as Americans.