Slitherine

As Illyria I'd been besieging an Athenian army Cappadocia for a few turns, and the food was running out. The Athenians decide to attack rather than starve, so a battle ensued and I was victorious!

Great I think, another step to building an empire and getting Athens to accept peace... except that the region was Commagene (who hates me intensely). There was no Commagene army any where near the siege so I'm somewhat irked that, not only have I lost my city, but I now have an enemy with access to my lands! They had tried many, many times to get through the mountains and a small force of MF and Cretan archers was holding them off indefinitely

Pocus, I have the files saved if you need them.

HHHmmm...Just a thought, if the besieged army contained any "mobile" army units and not the just standard regional troops.

In that case, if the Athenians had been at war with Commanege and two turns earlier had a peace proposal had been offered with Cappadocia being given back to the original owner (Commanege) then I think that is a possibility.

If that was the case then, at the start of the turn the deal is done and Cappadocia is given to Commanege, Athenian's army is still in the region, and then you fight Atenian's army, win and he is disbanded because there is no valid retreat location.

Yes that would be a possibility. Was the region with a 'war occupied' modifier?

I'm afraid I don't know.