Really? I post this for the second time:
the seceding States cried lustily, "Let us alone; you have no constitutional power to interfere with us." Newspapers and people at the North reiterated the cry.Individuals might ignore the constitution; but the Nation itself must not only obey it, but must enforce the strictest construction of that instrument; the construction put upon it by the Southerners themselves. The fact is the constitution did not apply to any such contingency as the one existing from 1861 to 1865. Its framers never dreamed of such a contingency occurring. If they had foreseen it, the probabilities are they would have sanctioned the right of a State or States to withdraw rather than that there should be war between brothers.
Ulysses S. Grant, Chapter 16: Discussing Secession, Personal Memoirs of U. S. Grant
After pretending to an illegal and unconstitutional secession the southern insurrection opened fire on US forces at Ft. Sumter, and declared war. That, as most honest people recognize, made legal considerations moot.
Grant was many things, but he was not a lawyer. Nor was Lee.
The legal opinion is Texas v. White. I recommend you look it up. It uses short words, so you will be able to follow it.
And I post this for the second time:
"We could not and ought not to be rigidly bound by the rules laid down under circumstances so different for emergencies so utterly unanticipated. The fathers themselves would have been the first to declare that their prerogatives were not irrevocable. They would surely have resisted secession could they have lived to see the shape it assumed." -- Ulysses S. Grant, Chapter 16: Discussing Secession, Personal Memoirs of U. S. Grant