Jiangnan area has been developed. What is the reason for this change?

There are indeed many reasons. Let me speak slowly.

First of all, from the perspective of agriculture and population tax. Since Xia and Shang Dynasties, the political, cultural and economic centers of China have been concentrated in the Yellow River Basin. The Chinese nation is a farming civilization and lives by water. At that time, the Yellow River basin was relatively flat and there were not so many floods. After Dayu's management, he mastered the water conservancy means to deal with sudden natural disasters. Therefore, in the Xia, Shang, Zhou, Qin and Han dynasties, the middle and lower reaches of the Yellow River, the so-called Central Plains, was the center and key area of grain tax. During the Spring and Autumn Period and the Warring States Period, the land of civilization in the south was just the so-called land of Jingchu, which is now the northern part of Hunan and Hubei, not very south. The kingdom of Wu in the south of the Yangtze River did not belong to the Central Plains family for a long time, and Fujian and Vietnam were regarded as barbarians and did not belong to the agricultural-intensive fat areas. The residents did not intend to dominate, and the strong did not intend to occupy.

Since the Han Dynasty, the imperial clan has been divided into different regions. With the general trend of unification, each region began to be systematically managed and developed. With the unremitting efforts of various princes and ministers, culture, education and advanced technology slowly began to spread to the south, and the south also developed. Even in the Eastern Han Dynasty, the south, especially Hunan and Jiangnan, had grown into a place producing food and taxes. In addition, the climate and geographical environment in the Yellow River Basin have gradually deteriorated, and the flood problem has not been eradicated, so the center of agriculture has gradually shifted.

Second, militarily. The middle and lower reaches of the Yellow River have been strategic places since ancient times. The terrain around Chang 'an and Luoyang is easy to defend but difficult to attack, and the strategic significance of the passes such as Hanguguan and Tongguan is great. It was here that Qin was able to advance and attack, retreat and defend, gradually gain advantages and then annex the world. On the other hand, the north is the area of northern minorities such as Rongdi. The so-called emperor's protection of the country, relying on the steep terrain and the Great Wall, as well as a large number of troops supported by the population, can effectively resist the invasion of the northern nationalities.

Of course, these military advantages are also double-edged swords. The so-called long-term separatist war in the north led to a large number of people gradually moving south, and the agriculture in the north was naturally hit hard by the war. The south has always been peaceful, and many literati and intellectuals who represented the advanced productive forces and elite culture at that time mostly moved south to avoid disasters. Therefore, during the Eastern Han Dynasty and the Three Kingdoms period, Jiangdong and Jingzhou were once full of talents, which made thousands of military forces in Sun Ce stand in the great cause of East Wu in later generations, while the nine counties in Jingzhou attracted Wei Shuwu's intrigue, and humanity won the Jingzhou world.

Generally speaking, the choice between the North and the South has brought more people to the South, and the value of the South has become more and more obvious. Of course, there are also sayings about Yin and Yang. It is said that there are three Long Mai in China, one is dominated by the Yellow River, the other is dominated by the Yangtze River, and the other is dug by Qin Shihuang in Guangdong. The situation is reversed. When the king of Long Mai in the north was weak, the dynasty in the south was stronger. When the southern king was weak, the dynasty turned to the north. Of course, just listen to this, don't believe it too much, or analyze it from the actual situation.

If you have other questions or don't understand, you can keep asking me.