As George pointed out and I'll highlight; slavery was only a catalyst for secession for some of the seceding states. The treatment of South Carolina by President Buchanan was a reason for more, and the topper was Lincoln's call for troops which tipped the balance for most Southerners to seceed and fight. Secession for slavery was not the typical formula, military coercion forcing secession was truer to the feelings of most Southerners.
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David Upton