"But I don't think most Founders would have understood the states in 1789 as "sovereign nations."
You are incorrect. Every state constitution contained a clause providing for the re-assertion of that states sovereignity in the event of an over reaching federal government.
Until the mid 1800's the right of succession was never seriously questioned. Many states had threatened succession including New York and Masachussetts.
Even during the Civil War many people even in the north asserted that succession was constitutional. In fact this was one of the issues which moved Lincoln to shut down many northern newspapers and jail writers and editors.
It is true that several of the founders argued against succession in these early cases but only from the standpoint of what was best for the state - never was it argued that succession was on it's face illegal or unconstitutional.
If the founders had argued that succession was somehow illegal they would have been undermining their own assertions against England
I'd have to check on that. Frankly I haven't read every constitution of that period.
Yet even if it were true that wouldn'tnecessarily be reflective of a Founders' views, since it is not a given that one was involved in the drafting of a state constition or a provision thereof. In fact, the Founders as a body were, I would argue reasonably, more nationalist than elites as a whole.
Until the mid 1800's the right of succession was never seriously questioned. Many states had threatened succession including New York and Masachussetts.
Your latter statement is certainly correct.
The doctrine itself, however, was a contentious one. I wouldn't paint it as settled as you do. There was a reason why no one actually carried out such a threat until 1860.
Even during the Civil War many people even in the north asserted that succession was constitutional.
Some certainly did.
New York City even had a major movement at one point for secession.
If the founders had argued that succession was somehow illegal they would have been undermining their own assertions against England.
Legally the cases are somewhat different.
The Constitition and the Articles of Confederation were voluntary pacts in a way that Crown control of the colonies never really was. And voluntary agreement creates moral and legal obligations which a coerced one, I might argue, does not.
But in the end secession is really the right to revolution which may be said to exist in natural law. If it succeeds it's a revolution. If it fails it's a rebellion. And the South, unlike the American colonists, failed.
And, may I say, thank God for that. However much I dislike what has become of the federal government and federalism, a world in which the South won the Civil War would be a much worse one all the way around.