I remember reading Eric Evans making the excellent point that while an historian can look at past events and come up with trends, he can't make laws about the progress of history. You can say, for example, that the trend in world government is away from religiously-backed monarchy and towards democracy, but you can't turn this into a law which says that it will always be that way.
By the same logic, the poor farmers of the southern USA, living hard, outdoor lives and used to using firearms, should have made far better infantry than northern city-dwellers.