content: The reason -- as it always is in war -- is economics, particularly power in economics. Industrialized North, agrarian South. North needed raw materials, South didn't want to give up it's mode of getting work done, which would erode their economic advantage (i.e. large slave population rather than the coal and steam powered model taking hold in the North.) Slavery was the primary mode of getting things done for as long as we have records of humanity. That was changing, and it displaced the old guard. They resisted. If there hadn't been a war, the change would have happened anyway: fossil fuels are easier to deal with than human labor.