The establishment of the Mongol Empire in the early 13th century brought about a major change in the international order of East Asia. Mongolia launched a massive attack on Jin. As a result, in 1214 Jin moved the capital to Kai-feng, south. Since then, Southern Song has stopped offering tribute and began to seek solidarity with the Mongol Empire. At about the same time, the Mongolian empire also recognized the need for cooperation with Southern Song. This is because he adopted a strategy of attacking through the territory of Southern Song in order to conquer Jin.
Mongolia invaded Jin in 1230 with a large army. At the same time, he demanded a way out to attack Jin to Southern Song. The Southern Song accepted the request and provided food to the Mongolian army. Attacked by Mongolia, Jin moved back to the south. Shortly thereafter Mongolia again proposed a alliance to Southern Song for the attack on Jin. Southern Song accepted the offer. Jin was finally destroyed by the Allied forces of Mongolia and Southern Song.
Since then, there has been intense debate over the restoration of Henan province within the Southern Song government. The policy of restoration of the northern territories was adopted. But the victory of Southern Song did not last long. The shortage of food and the counterattack of the Mongol army led to a great defeat of the Southern Song army. Duan-ping Ru-luo essentially destroyed the alliance between Southern Song and Mongolia. Since then, the two countries have entered a phase of war.