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?
The point when the Southern Liang Dynasty imploded.Prior to Hou Jing's rebellion,the south was actually in a prime position to conquer the North,given that northern China was divided between Western Wei(Northern Zhou) and Eastern Wei(Northern Qi). Hou Jing's rebellion destroyed the economy of the south and the fracture that resulted from this enabled the two Weis to conquer Sichuan and Southern territory north of the Yangtze. The territories the two Weis conquered were the most populated areas of the Southern Dynasty.
Prior to Hou Jing's rebellion,the Liang Dynasty could fight northern armies effectively(one of the largest pitched battles of the period,the Battle of Zhongli,which crippled Northern Wei, was won by Southern forces),and regularly did invade the north.One commander(Chen Qingzhi) was good enough to take advantage of the collapse of Northern Wei and capture the Wei capital with only 7,000 men,but he was not supported by the rest of the Liang army and was soon defeated after being confronted by an overwhelming Wei force.
The main reason why Southern Liang didn't conquer Western and Eastern Wei was because Emperor Wu of Liang(being an usurper himself)did not trust his best commanders,and often entrusted the control of the army to his incompetent relatives. The guy had no trouble finding effective commanders,it's just that he doesn't trust or use them.He only uses them whenever his incompetent relatives were defeated in battle,and the North counter-invades the south. After the North's counter-invasions were defeated however,he just returns command of these armies back to the same incompetent relatives who were defeated earlier.