Yes, the Civil War was about states' rights. Yes, the South was fighting for independence from what it saw as Northern political bias and Federal overreach. But all of those ancillary issues were about defending slavery as an economic and cultural institution.
Similarly, in the Confederate constitution, there were some provisions added that specifically restricted the ability of the states to handle slavery in their jurisdiction - e.g. the "right of transit and sojourn" (which essentially meant that free states were forced to respect the "property right" of any slave-owning citizen of another state who comes to reside there) - this, of course, was in response to Dred Scott.
And that's my perspective as someone who grew up with "war about slavery" as the default interpretation, only learning about contemporary states' rights arguments in later history classes at my (public Virginia) high school. (And then only learning from HN/reddit that "Northern aggression" / "states' rights" propaganda was even still a thing.)
I expect someone raised on that propaganda would be even more confused, likely enough to conclude that the speaker is lying or misinformed.
I actually think that preserving the Union is, in some sense, a more technically correct answer, since the South did not start the war [2]. The South declared its independence. The North started the war to prevent the South from gaining its independence [3].
As an aside. Independent of my actual interperatation of history, I think it would be highly unreasonable to deny citizenship to someone for holding a controversial belief that is shared by a significant portion of the country. Like it or not "states rights" is an American interpretation of the Civil war, even if it is one that is not historically accurate. And we really do not want the government to be in a position of dictating the "correct" history.
[0] You could argue about how trustworthy the North was. I would argue the main cause of the Civil war was that the South lost confidence that the North was (or would continue) to compromise on this issue.
[1] At least for a while. With hindsight, I am sure that slavery would have eventually been abolished in the South as well, and it probably would have been the North that was the primary political force behind it.
[2] Fine, they did open the attack of Fort Sumter when the Union refused to withdraw. You could therefor argue that the South did technically start the war, but I find that to be a bit of a stretch.
[3] As I mentioned above, this was the primary motivation of the North. The North, at the start, did not continue this sentence with "...so that they could abolish slavery".
More accurately, “states rights” was, at the time of the Civil War, a code word for protecting slavery that was revived during the Jim Crow era as a broader code for protecting segregation and systematic discrimination, and the faction which uses it that way has always been strong enough to play a major role in shaping the national mythology.
Further, Texas V. White was not a unamious decision. In the dissenting, Justice Grier wrote [1]:
>is Texas one of these United States? Or was she such at the time this bill was filed, or since?
>This is to be decided as a political fact, not as a legal fiction. This court is bound to know and notice the public history of the nation.
...
> I do not consider myself bound to express any opinion judicially as to the constitutional right of Texas to exercise the rights and privileges of a State of this Union
...
> I am not disposed to join in any essay to prove Texas to be a State of the Union when Congress have decided that she is not. It is a question of fact, I repeat, and of fact only. Politically, Texas is not a State in this Union. Whether rightfully out of it or not is a question not before the court.
This is reflective of a general political fact that sovereignty is not a legal question. Had the South won the war, we would not be having this arguement. It would be an undisputed fact that the Confederacy was a sovereign nation not subject to US law. It takes 2 sides to wage a war, and the North started the war because it found the South's political decision unacceptable [3]
Further, if we look at the post-war history, it was northern republicans who argued that secession was possible, as this is what allowed them to treat the former confederate states as conquered territory and therefore subject to direct congressional rule.
The southern Democrats (along with more moderate republicans) argued that secession was illegal, and therefore they were still full states in the Union, which would make laws such as the Reconstruction Acts [2] unconstitutional
[0] Not that it would have made any practical difference.
[1] https://www.law.cornell.edu/supremecourt/text/74/700#writing...
[2] As an unrelated matter, the North did repeal the Habeas Corpus Act 1867 in order to prevent the courts from ruling on the constitutionality of such reconstruction measures. Given the precedent set in Texas V. White, the southern states were likely correct that much of reconstruction was unconstitutional.
[3] That is not to say that the North was not justified in starting the war.
It's insane how often this is disputed when the Confederate states explicitly asserted this in their various declarations of secession. Mississippi's, for example, begins[1]:
> In the momentous step, which our State has taken of dissolving its connection with the government of which we so long formed a part, it is but just that we should declare the prominent reasons which have induced our course.
> Our position is thoroughly identified with the institution of slavery - the greatest material interest of the world. Its labor supplies the product, which constitutes by far the largest and most important portions of commerce of the earth. These products are peculiar to the climate verging on the tropical regions, and by an imperious law of nature, none but the black race can bear exposure to the tropical sun. These products have become necessities of the world, and a blow at slavery is a blow at commerce and civilization.
[1] http://www.civil-war.net/pages/mississippi_declaration.asp