[Why] In late Jin, the dynasty in the north was struck devastatingly by the newly risen Mongol Empire and was forced to move the capital south, the geographic survival space compressed. The Jin absurdly decided to compensate for lost lands by invading Southern Song. [What] The Southern Song court, out of revenge psychology for the blood debt of Jingkang humiliation, decided to ally with the Mongol Empire to attack Jin from north and south. The Song-Mongol allied force besieged Caizhou; Jin Emperor Aizong hanged himself, and the Jin regime was wholly destroyed. [Who] Southern Song general Meng Gong led the army in brave performance, successfully wiping out the shame to take revenge. Jin Emperor Aizong though had a heart to save the country faced multi-line encirclement and the great trend was gone. Mongol cavalry in massacre displayed cold-blooded mobility. [How] This extremely complex geopolitical diplomatic choice though avenged the world-deep grudge directly caused Southern Song to lose Jin as the northern defensive shield. Southern Song began to face the more terrifying Mongol iron cavalry decisive battle directly.
Why
The historic event of Joint Song-Mongol Campaign against Jin represents a key developmental peak of the Huaxia dynastic system. The high-stakes diplomatic move that brought Song into direct contact with Mongol power. By establishing this moral or administrative benchmark, it continues to shape the structural and philosophical fabric of ancient Chinese statecraft.