The Confederate constitution.  If secession was about states rights, you'd expect a different enumeration of federal v states rights in the Confederate constitution compared to the US one, with new limitations on federal government power. Except--that doesn't happen. The only new limit on federal power, the only new right states explicitly have, is slavery.

Look at the secession resolutions. They were all about slavery.

The south went to war for slavery, and the north went to war because South Carolina fired on a US fort.