Ok, so, you remember when there used to be the U.S.S.R. I don’t remember what all those initials stood for, but after it broke up, it became Russia, Belarus, Tajikistan, Turkmenistan, etc., etc., etc. It no longer that “Union” thing, nor “Socialist Republics” as some of the separate nation-states formed under a variety of different forms. Ok, so it happens. Nation-states break-up and then they have to reform.
Rewind our history, and 13 states break-up not “The Union” but the union of the nation-state, legally and factually, formed their own nation-state. AND most of those the remaining Congress convened and also formed a new nation-state because they had to, otherwise they were not a nation-state at all. Go and look at the Congressional Record for the Civil War. Read for yourself what they did and then start re-reading some of those amazing Supreme Court rulings that just make the hair on the back of your neck try to come around and choke you. They begin to make sense in the light of the laws under which the newly formed nation was founded upon, and the one to which the former states were readmitted not to the original “Union” but to one of a whole different legal context.