Secede. Specifically to secede over... [checks notes] the issue of wanting to be able to force all other states to actively take part in the slave trade regardless of their own decision to ban slavery internally.

It really is the most disingenuous shit trying to twist around the fact that the literal war itself was over the question of if states could secede or not (as the Union certainly wasn't principled enough to wage an abolitionist crusade for its own sake, and simply tacked that on to the more pragmatic concern of not wanting to lose a huge chunk of its territory to an explicitly hostile newly created foreign government that was led by people who'd already been waging a low-intensity war against the free states) to obfuscate that the actual secession part was caused by slave states being afraid they wouldn't be able to trample on the rights of free states anymore and wanting to secure their own federation where member states explicitly did not have the right to abolish slavery. The slavers were explicitly the anti-states'-rights side (and they'd spent the preceding decades strongarming free states into participating in the slave trade and allowing slavers to abduct free citizens from free states) except on the single issue of whether secession was possible or not.

Even the point of "they seceded to preserve their own 'right' to have slavery internally" is too generous to them, because they weren't even under threat of a federal abolition of slavery. They wanted to continue using the federal government to trample over free states' rights, and threw a tantrum when it looked like they might lose that power.