Pay the Mongols to attack the Sung? Is that possible? Otherwise your divide and conquer strategy appears best.If the Jin took the Mongol threat seriously they could easily have stopped it prior to 1210; let's not forget that Chinggis Qan was still paying annual tributes at the border market until 1210, the Jin just had to kill him when he came to present tribute. The success of the Mongol attack in 1211 was unexpected as the Jin literally neglected their northern defences. For some unknown reason, the Jin did not try to maintain the old divide and rule strategy and allowed a large state to rise in Mongolia under Ong Qan after the Tatar rebellion of 1196. The Jin might have thought a relatively unified state might have been easier to manage and kept rebellion under check. When viewed retrospectively, the Jin should have just actively supported the different factions in Mongolia so Chinggis never attain hegemony. A serious frontier defense preparation could also have easily prevented an invasion and maybe even crushing the Mongol insurgency.