The Civil War was about slavery. It had other causes, true, but slavery was a main cause. You can read the secession statements from several Confederate states and see that they, by name, cite slavery as a reason to secede. You can read them
here, and here is an excerpt from Georgia's:
"For the last ten years we have had numerous and serious causes of complaint against our
non-slave-holding confederate States with reference to the
subject of African slavery. They have endeavored to weaken our security, to disturb our domestic peace and tranquility, and persistently refused to comply with their express constitutional obligations to us in reference to that property, and by the use of their power in the Federal Government have striven to deprive us of an equal enjoyment of the common Territories of the Republic."