In the US Civil War, I've learned that France supported the south. If they had already banned slavery, why would they support the party fighting to keep it? Did they have any diplomatic or political reasons for doing so?
France placed a European ruler, Maximilian on the throne in Mexico for economic reasons. A united "United States" would have opposed such a move (and in fact, did, after the Civil War), under the Monroe Doctrine. France wanted the Confederacy as a "buffer state" between its protectorate in Mexico and the rest of the United States.