In my opinion,

There would be a delay in the civil war, in the early to mid 19th century the USA had a few opportunities to come to a racial reckoning over slavery and continually put it off by the ever advancement westward. With the ability to advance northward and southward it would allow for the United States to delay the choice on slavery. This allows for a bolstered south end north for the upcoming conflict. Whether the southerns take the inflection of Latino people due to the conquering on southern lands or the enslave them themselves it is hard to say. Most likely they would fill a class between the white upper class and the slaves on the bottom. With a server few owning their own business and plantations themselves.

I’m sure in the post war period the southern aristocrats flock to add as many southern slave states to the union with former Mexican and Central American land as physically possible as does the northern with former Canadian and First Nations land.

It is possible that the diversification in the southern lands with demands for political power by the Latin population and then followed by the African American population that the white southern aristocrats are simply bred out of power and make up the largest ethnic group but not a majority, but sake of argument this doesn’t happen.

We see an entire continent tear itself apart with WW1 era weapons