[jfb]

William McNeill, in Plagues And Peoples [1], makes the point that there are disease gradients that strongly influenced the movement of peoples and the development of culture, and that this gradient in China maps almost precisely to the rice/wheat border. Southern peoples had, by virtue of the ubiquity of disease vectoring mosquitos, an inbuilt defense against even massively superior Northern armies. This may be in large part why the Mongols took so long to conquer the Southern Song, and why Vietnam was able to successfully resist them.

[1] Highly recommended: http://en.wikipedia.org/wiki/Plagues_and_Peoples