How did the idea of
manifest destiny eventually lead to division within the union?

Basically, at one point in time while going westwards they reached states owned by Mexico, not just Native wildlands. There had to be a war then to conquer it to reach the sea. This divided people on whether or not people should fight Mexico. Another thing was that the expansion was at first mostly southern, which meant that more and more there were slave states, so they also divided on that and had to take in more northern places in to neutralize slavery influence.