Generally, it was their position that republics about the size of the individual states could survive, but that a republic on the size of the Union would fail. A particular point in support of this was that most of the states were focused on one industry—to generalize, commerce and shipping in the northern states and plantation farming in the southern. The Anti-Federalist belief that the wide disparity in the economic interests of the various states would lead to controversy was perhaps realized in the American Civil War, which some scholars attribute to this disparity. Madison himself, in a letter to Thomas Jefferson, noted that differing economic interests had created dispute, even when the Constitution was being written. At the convention, he particularly identified the distinction between the northern and southern states as a "line of discrimination" that formed "the real difference of interests".