How do I defeat the mongols?

The Mongol Invasion is just ridicilous. It is impossible to win againts them as Khwarazm. The fact that the whole event is so scripted is disgusting. They become hostile after the end turn with out a formal declaration of war and because of this the two vassals of Khwarazm are not part of the war. And everytime you defeat one of their armies they get two for free. You can't even pay them off because after a certain amount of turns they get two more armies and they become hostile again with out declaring war. And don't forget that they like to invite the Ghurids in just to make it more fun.