There’s a strong counter factual argument that can be made that if the Confederacy was allowed to exist seperately, then when we go into WW2 Hitler counts them as an Axis ally. Having a direct Axis ally on the continent probably means he can defeat the US, causing the Allied powers to lose the war. So: if the American Civil War had come to a draw, you can make a convincing argument the fascist powers (Germany, Japan, Italy, and a Confederate US) would dominate the world now. A scary thought.