According to the HistoryNet Article posted on September 2010 There are 5 important items that led the Civil War:
1. State´s Right: this fact was about the institution of slavery where the federal government was unique in regulating or completely ending slavery.
2. Slavery: In this case what led the Union to separate was the debate on the future of slavery.
3. Abolitionist Movement: By the 1830s, those who wanted to see that institution abolished within the U.S.A were becoming more strident and influential.
4. The Missouri Compromise: Additional territories gained from the U.S.–Mexican War of 1846–1848 heightened the slavery debate.
5. Raid On Harper´s Ferry Facts: The Harpers Ferry raid conducted by fanatical abolitionist John Brown and 21 followers in October 1859 is considered one of the major events that ultimately led to the American Civil War.