Given that the US was the last "western" country
to outlaw slavery, I cannot help but assume
it would have happened within the next two decades
with or without the war.
If the south had won the war
we would have had individual states
without very limited central government in the South.
Those individual state might have coalesced into smaller nations eventually.
The northernmost states from Pennsylvania, New York and New England
might have stayed together as a union.
Texas would have been it's own state
and maybe fought a war to take over Oklahoma and New Mexico, for example.
As the first world war came to a head
some states may have joined together
to support mutual defense
but it is unlikely that
all 48 states would have reformed
into one nation again.
We would probably have 4 nations instead:
the original southern states as a confederation
(Florida, Georgia, the Carolinas, Alabama, Mississippi, Kentucky, Tennessee, Virginia, DC) with a loose central government to provide national defense called the Confederation of American States (CAS)
the original Union of Northern states would keep the name (USA) and expanded west to Illinois
Texas and whatever states it could conquer would form a single nation not separate states like the CAS and USA would have. IT would simply be called "Texas"
California would form a nation with Arizona, Utah, Nevada, Oregon, Washington and probably the western part of New Mexico east of the rockies. It would be called the California Federation
-the bread basket region, everything from Colorado north and east to Iowa would likely become a separate nation unless Texas was very aggressive and conquered the whole thing, which it might.
Wisconsin would be up for grabs.
Which of these unions, confederations, and nations would have become communist?
After all, they would still run by men
who believe property is more valuable than people
The Northern group of states (USA) would be leaning
toward what you call "socialism" or "welfare states"
but none would be smart enough
to eliminate private ownership.
Sadly the vast majority of humans
don't get that you don't have to measure
the worth of human
by counting the amount of property
(s)he has managed to accumulate.
You don't have to measure a human being at all
you just need to show them respect
and they will be nice to you