To Abraham Lincoln, what did the statement, "Liberty and Union, now and forever, now and inseparable", mean?

To Lincoln the Union was perpetual and that once states were part of the Union they couldn't secede. I guess he must have overlooked the 10th Amendment. And, the Civil War wasn't about slavery no matter how often you're told that. The Civil War was fought to destroy the 9th and 10th Amendments to the Bill of Rights.