What if in this alternate timeline all border states that had slavery joined the Confederacy? This would include D.C as well, the capital of United States. What would the population difference be? Who would win the war? How would it even go?
It was of dubious legality, but Missouri and Kentucky did, and were represented in the CS Congress. Perhaps if the South didn't violate Kentucky's neutrality first, and had a more successful effort in Missouri, that could get two states. Maryland perhaps if the South, within the first four or five months, surrounded DC. There was a period of time where they could've done so early in the war.