The first issue is about Freedom, the slaves are still legal in south while north outlawed slavery, which causes tension between south and north. The North were fighting so hard to oppose slavery while the south were opt out on slavery.
Secondly the another issue is War. The slavery became a part of the large political crisis in U.S. History and the Civil War caused so much problems and was one of the bloodiest war in American history, because it was estimated that over 620,000 people died