[spoiler]Slaves[/spoiler]
http://www.nps.gov/fosu/learn/historyculture/upload/SLAVERY-BROCHURE.pdf
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It was an indirect cause. It was about the southern states to decide for themselves whether they should keep or free slaves. The northern states didn't want the country to split that's why they went to war. It wasn't a moral crusade to rid slavery in the country.