This isn't intended to become a wank, really...
Knowing full well that the Confederacy was on the defensive and didn't have designs on conquering the Union so to speak, what POD must be in place for the United States to become the Confederate States?
Civil War or at least the one IOTL is not a requirement. Perhaps a three way split of the nation (New England, South, North) and some disaster can result in the CSA getting lucky and occupying the North and West, or maybe the conquest takes place in a conflict post Civil War. An earlier Civil War with different aims? Some other earlier POD? An alternate Confederate President, more states seceding, and a more successful CSA Army could work too I suppose, but I'm leaving it up to you.
I don't know, I clearly don't have a lot of ideas or I wouldn't be posting this. So your challenge is to make the CSA the only America on the continent, by whatever means necessary.
Obviously, the outcome of whatever path you choose is expected. That should go without saying!
Thanks, guys.