Slavery was already starting to die in the South when the war began. R.E.Lee referred to it as the "peculiar institution" and recognized that it was costing them relations and business with Europe, and the Confederate Constitution featured more stringent restrictions on slavery than the US Constitution at the time. If ending slavery was the sole purpose of the war (it wasn't... retaining the union as a whole was a major factor), then fighting it was premature, and a blatant violation of SunTzu's principles of war.
I oppose going against Iran today for the same reasons. Give them a generation, and they will solve their (mullah) problem on their own, and won't be resentful for centuries for being forced to do so.