[Shadowstar]

Firstly, not only would that be way too complicated, but it would be impossible in simultaneous multiplayer games since players can't make decisions during other players' turns.

Frankly, I think my solution is the best one. Defending ships simply return to where they were if the enemy goes too far, and everyone's happy. There's no exploit since the attacker has to come back in order to start another combat, and every time they attack it would cost them movement points (and supplies). You wouldn't have immense combat maps either since this would all naturally keep the combat within a reasonable area. You could even throw in a rule that any ship can't attack again in the same turn if they retreated once during that turn already if you really want to cut down on combat log size.