Government decides what is illegal, and they use force to make sure people obey. Government is the entity which has a monopoly on force. That's what government is. That's what law is.
For revolution, civil war, or secession, any discussion of "legality" is simply misplaced. Either you are strong enough to get your way or you are not. There's nothing more to be said.
And the “issue of secession” was NOT resolved in the 1860s.
The right to secede exists regardless of whether the right was suppressed through the force of arms.
Secession IS legal under the original Constitution, inherently so. The Constitution would not have been ratified without the inherent understanding that if the _created entity_ of the federal government breached the contract, the states could withdraw from that contract.
The government in the 1860s simply enforced an illegal suppression of that legal right.