I'm besieging a fort of an independent tribe with my army of 113 power, defender at 31 power. It's taking a long time. While besieging said fort/city, I become allied with another faction. Allied faction moves army of 60 power into region I am besieging. Siege ends and he gets the region?
I had breached all walls and set my army to assault, no battle was fought and it is the allied AI region after the turn is over.