But then there were States not in this Union, were there not? You're wresting Constitutional language just as badly as any strange religious cult wrests Biblical language. Do sets and subsets ring a bell? Does legally requiring that every car on a public roadway be in good repair with insurance mean that no car can leave the public roadway, lol? Has every State even been guaranteed a Republican form of government? That certainly wasn't the case in former Confederate states during Reconstruction, and hasn't been since.
Reconstruction was the method used to attempt to ensure a Republican form of government.
Can’t say I entirely agree with the method, but that was the theory. There are several ways Reconstruction would not have been so traumatic.
One would have been for southerners to accept that they’d fought a good fight but lost and would now have to adapt to that fact. Blacks would just have to be accepted as fellow citizens. Congressinal Reconstruction was to a considerable extent a reaction to southerners who started, fought and lost a civl war, then expected they could go on pretty much as they had before the war. Doesn’t work that way.
The other would have been for Lincoln to have survived. We can never know, of course, but it seems likely he’d have led a Reconstruction that wouldn’t have left so much bitterness behind.
BTW, in just about every other civil war in history, the winners killed tens or hundreds of thousands of the losers in revenge. The USA executed one man.
Announcing that you're now a foreign country doesn't make it so.