Popular sovereignty is an important concept. It means to let people vote to decide an issue. As the country struggled with the expansion of slavery in the 1840s and 1850s, this idea was developed. The thinking was that if people could decide about the spread of slavery, it would lead to fewer conflicts. Thus, in the Kansas-Nebraska Act of 1854, people would determine if slavery would be allowed in these territories.
Unfortunately, it led to violent clashes in Kansas between pro-slavery and antislavery forces. These groups fought over the spread of slavery in Kansas. Thus the nickname “Bleeding Kansas” was developed.
In the Dred Scott decision in 1857, the Supreme Court ruled that popular sovereignty was illegal as it related to the spread of slavery. Even so, Stephen Douglas still believed this idea should be used to decide if slavery should exist in a territory.