I agree that they had the right to secede.
And they had the right to establish that right by the laws and usages of war.
What they don’t have the right to is to appeal the verdict of the laws of war. Its judgement is final.
And they had the right to establish that right by the laws and usages of war.
That sounds like circular reasoning. It sounds like you are saying they had the right to leave *IF* they had the power to win a war. In simple terms, "might makes right."
That is not how I understand natural rights. Your argument applies perfectly well to slavery. A slave cannot be free if he cannot overcome a superior force holding him in bondage.
This is rule by majority of bayonets.
What made the American experience exceptional was the founder's belief: That to secure these rights, Governments are instituted among Men, deriving their just powers from the consent of the governed . . .”
Of course, this did not survive the disaster at Appomattox.