[Why] After Northern Song collapsed in the Central Plains, Jin cavalry searched widely in the Central Plains for the Han royal blood. Prince Kang Zhao Gou, who had fled in time, became the only surviving pillar of the Northern Song royal family and had to rapidly rebuild the central authority. [What] Zhao Gou ascended at Nanjing Yingtianfu and rebuilt Song, called Southern Song. Facing the unceasing crazed Jin pursuit, he was forced to flee south all the way, and finally settled the capital at Lin'an in the south-of-Yangtze, with northern refugees migrating south in large numbers. [Who] Emperor Gaozong of Song Zhao Gou was an emperor who though held the name of founding sovereign was due to Jin-fear sentiment leaning toward partial-state retreat. Large numbers of loyal civil officials and northern southward-fleeing refugees together built a completely new homeland in the south. [How] The southward migration definitively consolidated Southern Song's partial-state-south-of-Yangtze rule pattern and promoted the Jiangnan economic circle's golden prosperity peak. It marked the definitive historic southward shift of Huaxia economic and cultural center.
Why
The historic event of Southward Migration and Rebirth of Southern Song represents a key developmental peak of the Huaxia dynastic system. The definitive shift of Chinese cultural and economic centers to the south. By establishing this moral or administrative benchmark, it continues to shape the structural and philosophical fabric of ancient Chinese statecraft.