In the era of cold weapons, it was almost impossible for southern China to unify the north, and the main reason was not because of the time and people, but because of the geographical advantage, which was a crucial factor enough to make the Northern Expedition from south to north in history fail miserably.
As we all know, the terrain of China's north and south can be described as very different, and the geographical form of the ancient north is relatively simple, with a plateau in the west and a plain in the east, which has both a strategic depth hinterland and a commanding height. In the south, the terrain is complex, divided into smaller pieces by rolling mountains, lacking strategic depth, and only the basins along the Yangtze River can be used to gain a foothold.
If the south wants to go north (usually the southern regime has Nanjing as its capital), there are only two ways to enter the army: one is from Jianghuai to Shandong, and the north is hebei to take the Beijing Division. The other way is from the Jianghan Plain into Henan, and the Henan soldiers are divided into two roads, west and north, respectively, targeting Chang'an and Taiyuan. Moreover, this northern expedition plan also needs a premise, that is, two-front coordinated operations, if one side loses, the northern expedition will inevitably abandon its previous achievements.
In contrast, the northern and southern expeditions were much simpler, not only did they have many routes to march, but also their goals were clear. Because the foothold of the southern regime is basically fixed in the middle and lower reaches of the Yangtze River, it is nothing more than a single political center Nanjing, at most counting a Hangzhou, that is to say, if you want to destroy the southern regime, as long as you occupy the Jiangnan region, the south will basically conquer.
In addition to the key factors of the terrain, for a long time, the population of the north was larger than that of the south, the grain production was greater than that of the south, and the number of horses required by cavalry was more than that of the south, which was also obvious advantages, so the difficulty of the south attacking the north was far greater than that of the north attacking the south.
The reason why Zhu Yuanzhang was able to succeed in the Northern Expedition was quite accidental. It was only by taking advantage of the serious division of the upper echelons within the Yuan court and the weak strength that zhu Yuanzhang had the privilege of sending troops to retake the north; if it were not for the partial disintegration of the Yuan court, the mutual seizure of power and the lack of time to take care of the rebellion in The Chinese region, Zhu Yuanzhang would have been difficult to complete the great cause of reunification in a short period of time.
In addition, after the Song Dynasty, the Zhao clan crossed south, so that China's economic center of gravity shifted south, and the Jiangnan region became China's main taxation, military, grain and grass base. Zhu Yuanzhang destroyed Chen Youyu in the east and Zhang Shicheng in the west, basically unifying the middle and lower reaches of the Yangtze River and sitting on the entire southeast, so that it had enough capital to chase after the Central Plains. Therefore, after the Song Dynasty, the rapid prosperity of the south also made up for the shortcomings of the strategic geography of the south, providing favorable conditions for Zhu Yuanzhang's Northern Expedition.
Of course, the above discussion is more applicable to the ancient cold weapon period, in the modern hot weapon period, although the geographical form is also very important, but more important is the advantages and disadvantages of weapons and equipment, followed by the strength of the will to fight, the focus of modern strategic defense will also shift from land defense to coastal defense and air defense, the probability of local operations will be greatly reduced.