In your opinion, what would be the effects of there not being a Mexican-American War on the United States?
Imagine that Mexico is much stronger and stable, has friends in high places (Europe), or a combination of both, so that a war against it would not be in America's best interest.
How would the situation between slave and free states develop? If war with Mexico is impossible, it's a given that war with Britain is too, so does Manifest Destiny just dies as a concept?