Good point, and partially correct so far as it goes. Thank you for raising that point.
The Constitution may be said, as southerners argued, to be a voluntary union between states. But it was also just as much a union between the men of those states. To the extent that it was the latter, secession becomes a difficult issue. What of those southerners who still felt that allegiance to that union to which they were, in a real sense, also parties to? And there were no few of those during the war, especially in Appalachia.
All of this is reflected in the structure of the Constitution itself. The Senate reflects the state understanding of the union. The House reflects the popular understanding.
Actually, the Constitution itself notes that it was established between the States so ratifying the same (Article VII), and can only be lawfully amended by three fourths of the States (which might one day actually be inhabited by a minority of the national population ;>). As Mr. Justice Thomas so astutely noted only a few short years ago:
[I]t would make no sense to speak of powers as being reserved to the undifferentiated people of the Nation as a whole, because the Constitution does not contemplate that those people will either exercise power or delegate it. The Constitution simply does not recognize any mechanism for action by the undifferentiated people of the Nation.
Clearly, the federal union is much less a union of the undifferentiated American people than it is a union of the several States.
;>)