The War of 1812 was determinative for the shape the United States would take over the course of the nineteenth century. To that point, many leaders had believed the key to fulfillment of America's Manifest Destiny lay with the conquest of British holdings to the north. Two things changed this. One was the Louisiana Purchase of 1803, which opened up vast new territories to the west. The other was America's inability to conquer Britain's Canadian colonies and thereby unite colonial North America under a U.S. government. In the end, expansion went westward, not northward.

As the century unfolded, the United States began to emerge as a growing force in the community of nations. The vast wealth of raw materials and lands attracted immigration, fueled development, and stimulated a growing transatlantic trade. Industrialization gradually sprang up, especially in the northeastern region of the country. Railroads were being built to transport America's products to markets and its people to ever more distant places. It was a period of prosperity and optimism, at least for much of the Anglo and European part of America's population. Two major groups were not integrated into the newly emerging political, economic, and social structures – those of Native and those of African heritage. These exclusions festered like open sores, igniting wars and social justice concerns that stretched throughout the century and beyond.