HINDUISM ~ Why shri Krishna did not kill "Jarasandha" ??

The army guys call it "tactical retreat" & not "running away from a battle". There are times when you have to be defensive and await the right time to attack your enemy, only when you are convinced that the dice will roll in your favour. Even if Krishna was a God, he couldn't save his fellow Yadavs who destroyed themselves and Krishna was a mute spectator. Evidently there were cirstances which even he could't control.