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Very few Chinese dynasties have managed to unify China from the South, with the Ming and the ROC being the only examples. However, as I understand, at the start of the Northern and Southern Dynasties, Southern regimes like Liu Yu's Song Dynasty were regularly able to defeat the North; with the situation reversing towards the end of the N&S Dynasties.
What was the last point the South could reliably defeat the North?
Why did the South fall behind the North? I suspect it had something to do with population disparity or scinicization?