Industrialization in the North and the growth and spread of slavery in the South were the two biggest economic and social trends in the United States from 1820 to 1861. These two developments go a long way toward explaining the cultural and political changes that occurred during the period. Most importantly, even though we now know that they led to the political crisis of secession and the Civil War, we can also see that the two developments were intertwined. Historians understand them as part of a broad "market revolution" that changed the economic foundations of the nation.

In the North, cities became centers of industry, state governments invested millions in building canals, roads, and other infrastructure, and a large working class, working as wage laborers, arose. In the South, slavery became increasingly entrenched, expanding rapidly through the Deep South. By the time of the Civil War, enslaved people were the most valuable economic asset in the nation—that is to say, the nation's economy was built upon the ownership of human beings. This was because of the importance of the textile industry to the market revolution. Cotton grown in the South with the labor of enslaved people was essential to the textile mills of the North, and for that matter Great Britain, where it was spun into thread and woven into cloth.

These developments spawned religious changes, such as the evangelical movement known as the Second Great Awakening. In the 1830s, working-class northerners and western farmers voted in large numbers due to the expansion of voting rights associated with Jacksonian democracy. The increased democratization of the United States went hand-in-hand with the Awakening, which stressed the individual's role in achieving salvation. It also spawned a series of reform movements, the most significant of which was the abolition movement. Though unpopular at first and always controversial even in the North, it grew in popularity as Northerners perceived that slaverholders were growing in political power.

The event that persuaded them of this was the Mexican War, which led to debates over the expansion of slavery into the western territories. This issue spawned a series of political clashes that led to the rise of the Republican Party in 1854. This party, rooted in the free labor ideology of the North, made stopping the territorial expansion of slavery its signal issue and led to the collapse of the two-party system that had held sway since the 1830s. When Abraham Lincoln, a Republican, won the presidency, Deep South states, perceiving that they could no longer use their political power to advance the cause of slavery, left the Union, beginning with South Carolina in December of 1860.