The interesting question is whether a Confederacy that was allowed to go peacefully would it have really poked the bear in the West and fought the US.....or if it would have turned its attention to the Carribean, Mexico, and Central America.
It’s impossible to imagine any US president convincing the public that people needed to fight and die to prevent slavery from spreading in Cuba or what have you.
It is an interesting question, but both sides quite openly stated that the issue was the western territories. As it was we fought campaigns all over the west to pacify the tribes. Had two countries competed for control of the west, it would have been even more interesting than it already was.
And had the confederacy competed in the Caribbean, that also might have led to conflict. It would make an interesting “alternate history”, maybe a whole series. I’d like to see a good writer take this on.
In any case, I don’t think we escape without war. War seems to be a common thread wherever people compete. This would be particularly true with two energetic countries so closely matched who proved subsequently to be quite ready to fight.