“So if they tell you the Civil War was about slavery...”
Here is what the North would NOT agree to: Expansion of slavery into the new states forming in the West. It also opposed returning slaves from the North to the South and letting Southern slave-owners bring slaved into the North.
But consider why Texas fought:
“The controlling majority of the Federal Government, under various pretences and disguises, has so administered the same as to exclude the citizens of the Southern States, unless under odious and unconstitutional restrictions, from all the immense territory owned in common by all the States on the Pacific Ocean, for the avowed purpose of acquiring sufficient power in the common government to use it as a means of destroying the institutions of Texas and her sister slave-holding States.
By the disloyalty of the Northern States and their citizens and the imbecility of the Federal Government, infamous combinations of incendiaries and outlaws have been permitted in those States and the common territory of Kansas to trample upon the federal laws, to war upon the lives and property of Southern citizens in that territory, and finally, by violence and mob law, to usurp the possession of the same as exclusively the property of the Northern States....
In all the non-slave-holding States, in violation of that good faith and comity which should exist between entirely distinct nations, the people have formed themselves into a great sectional party, now strong enough in numbers to control the affairs of each of those States, based upon the unnatural feeling of hostility to these Southern States and their beneficent and patriarchal system of African slavery, proclaiming the debasing doctrine of the equality of all men, irrespective of race or color—a doctrine at war with nature, in opposition to the experience of mankind, and in violation of the plainest revelations of the Divine Law. They demand the abolition of negro slavery throughout the confederacy, the recognition of political equality between the white and the negro races, and avow their determination to press on their crusade against us, so long as a negro slave remains in these States....
That in this free government all white men are and of right ought to be entitled to equal civil and political rights; that the servitude of the African race, as existing in these States, is mutually beneficial to both bond and free, and is abundantly authorized and justified by the experience of mankind, and the revealed will of the Almighty Creator, as recognized by all Christian nations; while the destruction of the existing relations between the two races, as advocated by our sectional enemies, would bring inevitable calamities upon both and desolation upon the fifteen slave-holding States.
By the secession of six of the slave-holding States, and the certainty that others will speedily do likewise, Texas has no alternative but to remain in an isolated connection with the North, or unite her destinies with the South.
For these and other reasons, solemnly asserting that the federal constitution has been violated and virtually abrogated by the several States named, seeing that the federal government is now passing under the control of our enemies to be diverted from the exalted objects of its creation to those of oppression and wrong, and realizing that our own State can no longer look for protection, but to God and her own sons—We the delegates of the people of Texas, in Convention assembled, have passed an ordinance dissolving all political connection with the government of the United States of America and the people thereof and confidently appeal to the intelligence and patriotism of the freemen of Texas to ratify the same at the ballot box, on the 23rd day of the present month.”
https://www.tsl.texas.gov/ref/abouttx/secession/2feb1861.html
I have addressed this particular point so many times, I need to just write up an essay and link to it.
In summary, I have found a lot of evidence to indicate this was just a lie told for propaganda purposes. There was never going to be any significant slave presence in the "territories." It was economically nonviable.
It also opposed returning slaves from the North to the South ...
It could oppose it all it wanted, but as that particular requirement was written into the US constitution in Article IV, Section 2. It was constitutional *LAW* that required escaped slaves to be returned. Look it up.
...and letting Southern slave-owners bring slaved into the North.
George Washington kept a slave plantation in Pennsylvania long after Pennsylvania declared itself to be a "free" state. I will also point out that telling American citizens they could not travel with their "property" was a violation of the privileges and immunities clause of the Constitution. States were required to accept the laws of other states regarding private "property."
But consider why Texas fought:
I looked up your source. It was dated February 2nd, 1861, which I believe was BEFORE the civil war. I'm not going to say you are trying to deceive us, but you may be unintentionally misleading people here. You are offering up a statement from the convention delegates regarding SECESSION, not fighting.
Texas fought because their sister states were invaded. Would you have fought if your sister states had been invaded or would you say "not my problem." ?
So you are offering up a reason alleged by a bunch of convention delegates as to why Texas left but this is different from giving a reason why they fought.
Do you have any statements from Texans who fought regarding why they fought?