You did not trust the Ongud, and therefore attempted to forge your own path in Jin territory. However, with scant knowledge of the area and few allies, you are prone to raids from Jin militias. Eventually, enough of your army is dead due to treacherous terrain, lack of suitable food in the Gobi desert, exhaustion, and raids that the superior numbers of the Jin forces are able to overwhelm your own when you finally meet.
You chose a non-historical decision! In real life, Genghis Khan trusted the Ongud. He made the marriage pact for a reason. Read more about how your decisions differed here.