I think that both issues are connected to one another and difficult to parse in terms of assessing how each caused the Civil War. Both issues are connected to one another. For the South, the issue of slavery was an issue of states' rights. The Southern states saw the issue of slavery as a practice of Southern states' rights. The South felt that the ability to practice slavery and ensure that slaves that fled could be obtained at any time from any given territory were extensions of the doctrine of states' rights. For the North, the issue of states' rights resided in their opposition to slavery. This, too, could be seen as the right of a state to object to slavery and not to have this freedom infringed upon by the South. In another sense, Northerners might have also seen the Southern desire for secession as an abuse of states' rights. Northerners did not see the Constitution as a loose compact, something enabling them to simply withdraw when Southerners saw fit. States' rights did not entitle the South to withdraw from the Union is an idea that Northerners observed during the time. The issue of slavery was broached with the the concept of states' rights, making it difficult to fully separate which reason had more bearing on the Civil War. In my mind, I think that a more logical approach would be to embrace the multiplicity of causes in terms of the bloodiest war in American History.