The division in America was not caused by the Civil War. It predated the US Civil war and was visible in England all the way back to the Middle Ages. It was an ethnic division found on the island of Britain that made its way to America in immigration and settlement patterns related to geography. This division was visible in Colonial times. For instance most members of prominent New England families returned to England and fought for the Parliamentary forces in the English Civil War while in Virginia the opposite was true with many going to fight for the king. And newspapers of the colonial era are full of pejorative opinions of each other.
Far from modern propaganda slavery wasn’t considered by most to be much of an issue either way until the 1820s when it became a shorthand for each side to condense all the reasons and arguments why they hated the other. But the hatred was already there.
The same division remains today. The heart of progressivism is in New England and one can see that the sections of New England and the Old South almost always are diametrically opposed in presidential elections. Movement of peoples within the states and foreign immigration has softened it some, but it’s still there and you can see it.
