Southern slavery would not have been extended to Mexico. Mexico was Christian, and the basis for negro slavery was that the people were pagan, and not civilized.
Counterfactuals are always very interesting, but the fact is that nobody knows what would have happened. We can look back in history and begin to understand how slavery came to be perceived as evil, when once it was seen as respectable. Something very significant happened for sure, where in the course of a hundred years, what was a nearly universal practice became virtually wiped out of existence everywhere.
Christianity was a factor, but certainly was not the only one, for Christians found many ways to justify it throughout the history of Christendom. And most certainly, as Scott has already pointed out, Irish and Catholics made for very good slaves as far as other Americans were concerned, so no doubt brown skinned ones from the Latin countries would have been a good source too.
But I of course do not know how things would have turned out if the Civil War had not been fought, and slavery was allowed to continue its natural course. My main focus here was to counter the rosy optimism of libertarian thought which somehow sees progress as natural, even if obvious evil goes unopposed.
Even if I have previously overstated my case that slavery did not exist outside of the plantation south, the main point is that this is something of a very different character than what existed elsewhere in the British Empire and America, and there is no reason to believe that it would have inevitably been legislated out of existence due to the inevitability of progress.
The Civil War was a very big part of the anti-slavery drive. It has been a very big part of the necessary myth-making in which slavery is portrayed as an evil Just as much as British mastery over the seas, and the British and Christian moralism that decided that slavery was intolerable, the story of the Civil War in America has defined for us where good people stand when it comes to slavery.
The world did indeed progress socially during the nineteenth century on the issue of slavery. But progress is not inevitable. There is no reason to think the ageless institution of slavery would have fizzled out anyway, if the battle had not been fought. Indeed, without the battle, it is very easy to envision a very different kind of world.
That was my only point.
