Lets say that during the American Civil War, some European countries choose to support one side, and some support the other, leading to a World War.
In Norton's post, Britain and France support the Confederacy, with Russia and Germany supporting the Union. OTOH, it could be than France chooses to support one side, and Britain the other, and then Russia and Germany pick sides.
What nations would have been most likely to pick what sides, and who else would get dragged in by the tangle of European alliances?
What would a WWI in the 1860s have been like, and how would it have progressed?