Secession
Issues over how to explain secession quickly became important for framing historical narratives around the Civil War. The legality of secession, and a State's right secede from the Union, were central the the argument that the war was over issues of State's rights and soveriegnty. If States were truly within their rights to withdraw from the United States, than it was the North who acted aggressively in perpetrating the war. If secession was only a percieved right by some and unfavorable to most, soveriegnty and protecting States' rights falls apart as a valid justification, and therefore it is the South which should be held responsible as the aggressors of the conflict.
In addition to arguments over if the Southern States had the right to secede--and just as controversial--were the descriptions of why the Southern States seceded. Were they seeking to protect themselves and their rights, or was the issue of slavery perhaps more central to the cause?