By your own oft-repeated standard, states are still free to leave the same way they entered meaning with the approval of Congress.
It was true then, it's true now.
JCBreckenridge: "Lincoln, in forming an army and invading the Confederacy border committed an act of war."
None of which happened until after the Confederacy started war at Fort Sumter, and formally declared war on May 6, 1861.
“By your own oft-repeated standard, states are still free to leave the same way they entered meaning with the approval of Congress.”
Half a million American men died proving this is not so.