The states were no longer united, thus the United States had ceased to exist, except in the aggressive minds of Lincoln and his staff. How you figure Washington DC is an entity that owns things is beyond me. Collective property is not by definition the sole property of on party. The North was already way ahead on possession of the collective property, but they claimed a right to all of it. Very aggressive and hostile.
Since its inception the United States has never “ceased to exist”. There was that one short period of southron temper tantrums but we fixed that ;’)
Treating what until recently they'd thought of as their country now as an enemy or as some nothing to be despoiled at will produces bad blood and an inevitably hostile reaction from the rest of the country.
If you want to learn from experience, look at recent arrangements in Canada and the UK where both sides worked together to create conditions for possible Quebec or Scottish independence.
But I suspect the point of all these arguments is precisely not to learn anything. It's to go around with the same "f-you" attitude that worked out so poorly for everyone the last go-around.
It's not the goal of independence that some people actually want. It's all the adolescent drama of slamming the door on the way out and shouting parting obscenities. It's not the actual change in one's status that's valued, but the feeling that one was right and justified all along in whatever one does.