This article concisely summarizes the historical evidence that the South did, in fact, secede from the Union primarily in order to perpetuate slavery: https://www.theatlantic.com/politics/....
Here's a list of the Confederate state legislatures' declarations of their reasons for seceding: https://www.civilwar.org/learn/primar....
For Alexander Stephens' "Cornerstone Speech" of 1861, see https://www.civilwar.org/learn/primar....
In 1894, Confederate commander John Mosby wrote to a former fellow soldier: "I've always understood that we went to war on account of the thing we quarreled with the North about. I've never heard of any other cause than slavery." (http://www.encyclopediavirginia.org/M......) In 1907, he wrote to another fellow former Confederate: "I am not ashamed of having fought on the side of slavery." (http://tinyurl.com/kxu9aag)
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