Westward expansion divided the country over whether a new state or territory should be a free or allow slavery.  Also, the Supreme Court ruled that slave Dred Scott was never free, even though his master had taken him into free territory.  Abolitionist John Brown's raid at Harper's Ferry, Virginia divided the country even further, which led to the Civil Warn (http://www.history.com/topics/black-history/slavery).

Dred Scott (www.pbs.org)

John Brown's raid