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.