This is no different than what happened in the Civil War.
Each side had a list of grievances. A big one on one side, was the preservation of slavery in perpetuity. The other side wanted to preserve the unity of the nation as a whole.
You are right that Mexico was not fighting to ban slavery. But it was clear that eliminating slaver was still in their roadmap. Texas had only gotten an extension.
Similarly the election of Lincoln wasn't an immediate threat to slavery in the Confederacy. But the Confederacy still was motivated to head off the long-term threat to their economic system.
So the fact that Mexico wasn't fighting to end slavery, doesn't contradict the fact that Texans were fighting to preserve it.
Opposition to that act was, "We're not going to create new states with slavery." It wasn't, "We're getting rid of slavery in places that already have it." Which meant that it was not a direct threat to existing slave states. (Though there was a prospect that maybe, some day...)
Lincoln didn't like slavery. But he wasn't trying to ban it. It wasn't until the war had been going on for nearly two years that he signed the Emancipation Proclamation. Which basically said, "If you're still in rebellion a couple of months ago, we're going to free your slaves." It didn't eliminate slavery for any slave state that was part of the Union. It was an incentive to surrender. It was 2 years after that until the 13th amendment was passed.
It is very unlikely that slavery would have been banned during Lincoln's time in office, if the Civil War hadn't happened.
Was there a long-term prospect of a threat? Obviously. But there was no immediate threat. And so the fight wound up with protecting slavery on the one side, and asserting the indivisibility of the nation on the other side.
This is exactly parallel to the Mexico-Texan issue. Slavery was not being banned in the rebelling states.
> Opposition to that act was, "We're not going to create new states with slavery."
Pretty sure you’re arguing in bad faith.
A group of anti-slavery people organized into a political party and elected a man who gave a speech declaring that slavery itself needs to be abolished.
Pretending that Lincoln wasn’t going to attempt to abolish slavery is just as bad as pretending the civil war was about states rights (states in the confederacy had mandatory slavery; no right to choose).
No, I'm not arguing in bad faith. You are oversimplifying and mischaracterizing someone whose complex views tried to balance morality and the ugly facts on the ground.
Lincoln's actual beliefs can be seen in his own words. Let's take, for instance, the first of his seven debates with Stephen Douglas.
In this debate, Lincoln made it clear that he morally opposes slavery. He therefore opposed allowing it to spread. But he pointed out simply trying to suddenly eliminate it would create even worse problems. He therefore did not support abolitionism. He even supported returning slaves to slavery if they escaped to a free state. He only supported containing the practice, and hoped for a better solution over time.
This balancing act is made clear in all of his speeches before he was President. It is even visible in the Emancipation Proclamation. It was only after years of war that he came to support the hard-line stance represented in the 13th Amendment. It is extremely unlikely that he would have gotten there without having been hardened by the Civil War.
There really was no immediate threat to slavery. It was only a long-term, potential threat.
Here is an exact passage from his speech laying all this out.
"When Southern people tell us they are no more responsible for the origin of slavery than we, I acknowledge the fact. When it is said that the institution exists, and that it is very difficult to get rid of it, in any satisfactory way, I can understand and appreciate the saying. I surely will not blame them for not doing what I should not know how to do myself. If all earthly power were given me, I should not know what to do, as to the existing institution. My first impulse would be to free all the slaves, and send them to Liberia,-to their own native land. But a moment's reflection would convince me, that whatever of high hope, (as I think there is) there may be in this, in the long run, its sudden execution is impossible. If they were all landed there in a day, they would all perish in the next ten days; and there are not surplus shipping and surplus money enough in the world to carry them there in many times ten days. What then? Free them all, and keep them among us as underlings? Is it quite certain that this betters their condition? I think I would not hold one in slavery at any rate; yet the point is not clear enough to me to denounce people upon. What next? Free them, and make them politically and socially our equals? My own feelings will not admit of this; and if mine would, we well know that those of the great mass of white people will not. Whether this feeling accords with justice and sound judgment, is not the sole question, if, indeed, it is any part of it. A universal feeling, whether well or ill-founded, cannot be safely disregarded. We cannot, then, make them equals. It does seem to me that systems of gradual emancipation might be adopted; but for their tardiness in this, I will not undertake to judge our brethren of the South.
"When they remind us of their constitutional rights, I acknowledge them, not grudgingly, but fully and fairly; and I would give them any legislation for the reclaiming of their fugitives, which should not, in its stringency, be more likely to carry a free man into slavery, than our ordinary criminal laws are to hang an innocent one.
"But all this, to my judgment, furnishes no more excuse for permitting slavery to go into our own free territory, than it would for reviving the African slave-trade by law. The law which forbids the bringing of slaves from Africa, and that which has so long forbid the taking of them to Nebraska, can hardly be distinguished on any moral principle; and the repeal of the former could find quite as plausible excuses as that of the latter."
Each side had a list of grievances. A big one on one side, was the preservation of slavery in perpetuity. The other side wanted to preserve the unity of the nation as a whole.
You are right that Mexico was not fighting to ban slavery. But it was clear that eliminating slaver was still in their roadmap. Texas had only gotten an extension.
Similarly the election of Lincoln wasn't an immediate threat to slavery in the Confederacy. But the Confederacy still was motivated to head off the long-term threat to their economic system.
So the fact that Mexico wasn't fighting to end slavery, doesn't contradict the fact that Texans were fighting to preserve it.