[Age of Reason]

ratification was not revokable and joining the Union removed any "right" to secede.

Well there you go: The War was about the right of the states to secede--and not about slavery.

For having slaves was legal, but secession was not.

[Age of Reason]

More accurately:

The War was about the illegality of the states to secede--and not about slavery.

For having slaves was legal, but secession was not.

[justshutupandtakeit]

For the North that was precisely the question. For the South it was entirely about slavery that was the only "right" it was concerned about.