I ain't really got a dog in this hunt, but what you said in your post there ... incorrect. The Confederate States did not seek to "overthrow America." They sought to leave, like a Brexit, from the Union of States. Before the Civil War the federal govt. was a much less significant entity, most people considered themselves a citizen of a State first. Without getting into debating over what the Civil War was even about, the Confederate States did not seek to overthrow or occupy or rule the USA. They just wanted out. They wanted their own country. The Northern States wouldn't allow them to leave, and the rest is history. What Constitutional authority the Northern States were relying on to claim that no State could ever leave once they joined, I have no idea.
they sought to leave to retain their slaves. all you need to do is look at the confederate constitution. Specifically look at the amendments made.
https://en.wikipedia.org/wiki/Confederate_States_Constitution#Slavery
some key points are:
Article I Section 9(4)
No bill of attainder, ex post facto law, or law denying or impairing the right of property in n**** slaves shall be passed.[14]
no bills or laws can be passed to remove n****'s who are property.
Article IV Section 2(1)
The citizens of each State shall be entitled to all the privileges and immunities of citizens in the several States; and shall have the right of transit and sojourn in any State of this Confederacy, with their slaves and other property; and the right of property in said slaves shall not be thereby impaired.
If you bring you're slaves to the confederate states, they remain slaves.
Article IV Section 3(3)
The Confederate States may acquire new territory; and Congress shall have power to legislate and provide governments for the inhabitants of all territory belonging to the Confederate States, lying without the limits of the several states; and may permit them, at such times, and in such manner as it may by law provide, to form states to be admitted into the Confederacy. In all such territory, the institution of n**** slavery as it now exists in the Confederate States, shall be recognized and protected by Congress, and by the territorial government: and the inhabitants of the several Confederate States and Territories, shall have the right to take to such territory any slaves lawfully held by them in any of the states or territories of the Confederate states.
If the confederate states take territory, slavery is now legal there.
Anyone who does not think that slavery was the core part of why the confederate states were founded is ignorant at best, and purposefully disingenuous at worst.
Also the Cornerstone speech written by the only vice president of the confederacy:
Stephens's speech declared that disagreements over the enslavement of African Americans were the "immediate cause" of
secession and that the
Confederate constitution had resolved such issues, saying:
The new Constitution has put at rest forever all the agitating questions relating to our
peculiar institution—
African slavery as it exists among us—the proper status of the n**** in our form of civilization. This was the immediate cause of the late rupture and present revolution. Jefferson, in his forecast, had anticipated this, as the "rock upon which the old Union would split." He was right. What was conjecture with him, is now a realized fact. But whether he fully comprehended the great truth upon which that rock stood and stands, may be doubted. The prevailing ideas entertained by him and most of the leading statesmen at the time of the formation of the old Constitution were, that the enslavement of the African was in violation of the laws of nature; that it was wrong in principle, socially, morally and politically. It was an evil they knew not well how to deal with; but the general opinion of the men of that day was, that, somehow or other, in the order of Providence, the institution would be evanescent and pass away. [...] Those ideas, however, were fundamentally wrong. They rested upon the assumption of the equality of races. This was an error. It was a sandy foundation, and the idea of a Government built upon it—when the "storm came and the wind blew, it fell."
[4]
Our new government is founded upon exactly the opposite ideas; its foundations are laid, its cornerstone rests, upon the great truth that the n**** is not equal to the white man; that slavery, subordination to the superior race, is his natural and normal condition. This, our new government, is the first, in the history of the world, based upon this great physical, philosophical, and moral truth. This truth has been slow in the process of its development, like all other truths in the various departments of science.
May we not therefore look with confidence to the ultimate universal acknowledgement of the truths upon which our system rests? It is the first government ever instituted upon the principles in strict conformity to nature, and the ordination of Providence, in furnishing the materials of human society. Many governments have been founded upon the principle of the subordination and serfdom of certain classes of the same race; such were and are in violation of the laws of nature. Our system commits no such violation of nature's laws.