"Clearly what Lincoln is saying isn't that states can unilaterally renounce the Union, but that any people, anywhere, can revolt against an existing government and claim as much territory as they can hold. In this case Texas had revolted against Mexico and succeeded, through force, in holding territory. The question was whether this territory extended to the Rio Grande or only to the Nueces."
It that not talking out both sides of your mouth. It's ok for Texas to leave Mexico but the Southern states can't leave the US.
Huh?
It's ok for Texas to leave Mexico but the Southern states can't leave the US.
I'm not up on the Mexican Constitution of the time, but I'm willing to bet that there was no mechanism outlined in it for Texas to unilaterally secede. Their right was a natural right in the Lockean sense. Similarly, the 13 colonies had a natural right to rebel against England, but their action was utterly illegal under British law. The founding fathers fully recognized this and got busy fighting for their independence rather than making a legalistic argument.
Southern states had a natural right to revolt against the United States, not a constitutional one. They aren't the same thing.