Slitherine

When I'm allied with two people who are fighting each other and one attacks the other, if my unit is the strongest in the defending tile it will be assaulted by my own ally.

I was thinking that maybe having an ally stacked with an enemy should prevent attacking that tile (It certainly should prevent assaulting a city, which happened once -- my trooper ended up killing one of my own allies) until you declare war on your previous ally, or get them to disassociate with the guy who's stacking units with them.

That would also clean this issue up.