the North, states banned slavery and saw it as a backwards and "peculiar" institution. In the South, slavery was a key aspect of the economy, as it was slave labor that built the wealth of the South through crops like cotton. As this issue grew more divisive, both sides feared the opposing side gaining a majority in Congress, as this could lead to the passing of federal laws either to preserve slavery or to ban it across the United States. One way to keep this divisive issue from tearing the country apart entirely was to maintain a balance between the number of free states and the number of slave states in the U.S. This would mean that there would be an equal number of free and slave state senators, and no federal laws about slavery could easily be passed.

With the purchase of the Louisiana territory from France in 1803, the United States gained an enormous piece of land and settlers began to head west from the coast to settle this land. Internal improvements, such as roads, canals, and eventually the railroad, would help to speed this process up. As more people moved into the territories making up the Louisiana Purchase, there were efforts to organize the territory into states. Between the years of 1816 to 1819, for example, four new states were added to the country. Two of these states (Indiana and Illinois) were free states, while two were slave states (Mississippi and Alabama). There was an effort to maintain this balance in order to preserve the country despite disagreement over the institution of slavery.

In 1820, Missouri would apply to enter the United States as a slave state; however, this would upset the balance between free and slave states. Eventually a compromise was reached that would allow Missouri to enter the Union as a slave state, while Maine would be created as a free state. They also established the 36-30 latitude line as a division regarding the admission of future states. Any future new states north of the line would be free, and any future new states south of the line would be slave.

Following the Mexican-American War, the United States gained more land to its west. The following gold rush in California would quickly establish California as a state. Under the Compromise of 1850, California would be admitted as a free state, but to appease Southerners, a new and stricter fugitive slave act would be created as well. There also came a growing desire to build a transcontinental railroad across the country to link California and the west coast with the rest of the United States. As a result, pressure rose to organize the territories of Kansas and Nebraska so that the railroad would be built through them. This, however, would upset the balance between free and slave states, as both Kansas and Nebraska were above the 36-30 line. In attempt to solve this problem, Illinois politician Stephen A. Douglas proposed the idea that the status of slavery in Kansas and Nebraska would be decided by popular sovereignty—or by allowing the settlers there to vote on the issue.

The Kansas-Nebraska Act, which followed Douglas's proposal, was passed. It also repealed the 36-30 line established by the Missouri Compromise. The result, however, did not bring a peaceful solution. The Kansas-Nebraska Act was hated by northerners and celebrated across the South. It also led to the settlement of these territories, particularly Kansas, by both pro-slavery and anti-slavery settlers, each trying to help determine the status of slavery in these territories. The result was violence and death between the two groups as they came into close contact. This would lead to the use of the term "Bleeding Kansas" to describe the crisis that would occur there. Some historians also cite this outbreak of violence as the true start of the Civil War conflict.

While the Kansas-Nebraska Act attempted to help settle and organize land in the west, and bring about a solution to the question of whether or not Kansas and Nebraska would be free or slave, it instead brought about conflict. This deepened already heated divisions between the North and South regarding the issue of slavery. It is for this reason that the Kansas-Nebraska Crisis is often mentioned when examining the causes of the American Civil War.

The Kansas-Nebraska Crisis grew out of the Missouri Compromise of 1820.  According to the Missouri Compromise, the admission of new states into the Union would be staggered, so that the number of free states and slave states would be the same, giving neither the numerical advantage.  In 1854, a similar situation arose.  In discussions to build a transcontinental railroad, the question of the route the railroad would take took on great importance.  It was believed that if the track was laid through Southern states and territories, the building of the railroad would aid the spread of slavery to the West.  To counter this, Stephen Douglas, an Illinois senator, proposed that the railroad be built along a Northern route.  This debate ultimately resulted in the Kansas-Nebraska Act of 1854, which stipulated a third route through newly-formed territories:  Kansas and Nebraska.

With this solution, the question of whether these territories would be slave territories or free territories became a serious issue.  If both were made slave territories, the North would object, while the South would take issue if both territories were admitted as free territories.  In the Kansas-Nebraska Act, Douglas proposed that popular sovereignty be exercised.  Prevailing practice in the given territories would determine whether they would be slave territories or free territories.  The main problem with this is that the Missouri Compromise prohibited slavery above the 36 30' line, and the Kansas-Nebraska Act negated this, leaving the choice up to the population rather than the government.  The Southern Democrats and Whigs quickly passed the act, while the Northerners were greatly angered by it, fearing the spread of slavery into the North.  To force the issue, thousands of Missourians flocked into Kansas in the hope of making it a slave territory, while abolitionists moved to counter this.

The two populations in Kansas could not agree on a territorial government.  After the two sides could not agree on a joint state constitution, the proslavery contingent in Lecompton, Kansas drafted a constitution and submitted it for ratification.  Millard Fillmore accepted the constitution and welcomed Kansas into the Union as a slave state.  Ultimately, this episode brought the Sectional Crisis to the brink of Civil War, which broke out only four years later.