Victoria II: A House Divided adds 1861 as a new starting point to Victoria II. This puts you right at the beginning of the American Civil War, meaning you can pick the Confederate States of America as your faction. This expansion is about more than just the Civil War, though. It also gives us a new user interface, better revolutions, new ways to initiate wars, and more reasons to play as an uncivilized nation.
If you're a Civil War buff then you'll be drooling over the contents of this expansion, but even without the Civil War content there are still enough interesting changes to make this one of Paradox's best expansions.