” If you want to say that there is no right to secession, then Virginia could not have left the Union.”
Which of course was Lincoln’s position right from the start. He argued that there was no secession and that the CSA didn’t exist.
This is why he always refused to meet with CSA peace delegations. To do so would be to recognize the CSA. It’s also why Lincoln didn’t ask Congress for a declaration of war. He was suppressing rebellion, not waging war against another nation.
Lincoln’s position was that he was dealing with a Whiskey Rebellion writ large. “Combinations” had managed to seize control of entire states. But those states had never left the union because secession wasn’t possible.
Charles Francis Adams Jr. discussed the legality of secession in his circa 1900 “Shall Cromwell Have a Statue?”. He concluded that both sides were right. That the original post-Revolutionary conception of the union allowed for it. But that sometime later, maybe around the 1820s, a view had grown that the union was perpetual. In neither case was this explicitly spelled out in law. Adams Jr was the son of John Quincy Adams, the great-grandson of John Adams, and had been a colonel in the Union Army. So he if anything he was arguing against interest by stating the argument for secession was valid. As was the argument against.
No country gave the rebellion diplomatic recognition.
One would think that the Declaration of Independence would completely clarify the issue for all concerned, seeing as how it was the basis of our own independence from the United Kingdom.