New Fire control

I was thinking about a new type of fire control, I think it should be called "Defensive" fire control, and if activated, it would operate like fire at will, however, it would also fire on enemy units that were attacking your allies, e.g. if you've got a battleship outside Rome, so you can shell the city so your ally can take it, but suddenly your ally's convoy that is travelling nearby is attacked by a foreign cruiser, then this fire control would mean that your battleship would start attacking the cruiser, because it was damaging an ally's unit.