If I remember correctly, the S. Song *lost* to the Mongolians in the end who used their control of N. China to put together a N. Chinese navy that could go toe to toe with the S. Song navy and a N. Chinese army with military engineers that could match Song military technology. Practically speaking, barbarians that control the north are dangerous as they have access to Chinese military technology, governmence and tactics which negates much of the advantages Han Chinese have over the "barbarians". Barbarians with Chinese technology, tactics and governmence are a massive military threat as history shows. There has never been a case in Chinese history where the Southern Chinese have been able to militarily defeat a N. China whether controlled by barbarians or Han Chinese or even successfully defend against them (in the long run). Staying in the south and defending against the north has always been a losing tactic in the end.
Three Kingdoms - Sun Quan (S.) lost to Wei (north). Wei was Han Chinese but it was still N. vs S.
E. Jin->S. Chinese Kingdoms - lost to the founder of the Sui. dynasty who was from the north.
S. Song - lost to the Mongolians controlling a N. Chinese army.
Ming - were driven south by the Manchurians. Tried to defend the south. Were eventually unable to hold the south either.
There's not much a success rate here defending the south against the north in the long term. Typical sequence of events:
- China gets fragmented into N. and S. due to internal conflict and/or barbarian invasions.
- N. easy to unite/conquer due to flat plains.
- N. reach Yangtze river, meet S. Chinese fleet, get owned.
- stew for a few decades to a century. Eventually get united and get a decent fleet together.
- manage to get over Yangtze.
- slowly beat their way south down the river valleys and mountain ranges. Takes a while as terrain is rough but eventually gain control of the South.
rinse, repeat.
Besides as others said, N. China is always considered the ancestral heartland of China. The South was until recently considered second best and less civilised. No Chinese emperor would abandon the north. Those who did are considered by history to be corrupt cowards.
Dann said:
So abandon the north to them Mongols! Go the way of the Southern Song and hide behind the Yangtze, but evolve into a naval superpower and be like Britain, only by a few centuries earlier.
With colonies all over Southeast Asia, the Middle East and East Africa, there would be enough money and resources to offset the loss of northern China. Do this one bit further and they'll reach the Americas. I think the armies will go crazy with the Aztecs and the Incas too, just like Cortez and Pizarro. Gold is gold for any materialistic civ with a military edge. Especially if they see the natives having common utensils made out of the stuff.
With further research, they might just get enough technology to eventually strike back at the Mongols. It might take a while, but is certainly better than hiding behind the Great Wall and stagnating.