The Kansas-Nebraska Act of 1854 gave settlers the right to vote on whether slavery would or would not be allowed before a territory became a state. This act was based on Senator Stephan Douglas' concept of popular sovereignty. Later in 1859, the US Supreme Court in the Dred Scott decision took away from the US Congress any power to regulate slavery.
Stephen Douglas's optimistic idea that the people of each new state should vote whether to be slave or free.
Because these states would be voting one at a time, this gave every bully-boy in America the opportunity to descend on a single state and cause maximum mayhem.
One cause was the interference of northern and southern settlers on the issue of slavery.