Answer:
The fundamental cause of southern secession (and ultimately the Civil War) was the US's inability to solve slavery at the national level. ... When the northern states were threatened by the War of 1812, they considered secession. When South Carolina was threatened by a tariff, they attempted to nullify the law.
Explanation:
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https://history.stackexchange.com/questions/553/why-did-the-southern-states-secede-from-the-u-s
