My idea on this is thus:
Attacker attacks you. S/he conducts opening fire, removes casualties, conducts normal fire, removes casualties, at this point, the attacker has the option to attack or retreat. S/he decides to attacker. Now defender has the option to retreat and decides the battle’s lost, best to conserve for a later day.
Any infantry, armor or fighters the defender has may retreat one space to a friendly territory. They must all retreat to the SAME place (yes, even the fighters, no separation.)
Any artillery or bombers have to be left on their own, but may inflict damage on the attacker in subsequent rounds. Idea being, artillery is heavy and not easy to move, and bombers are not defensive units, they also don’t magically launch when the base is under attack, more likely they are on the ground and their crews are using pistols to defend the base.
No defender retreat option from naval attacks. (Except fighters.) Naval warships traveling at 6-8 knots are not going to out run anything, especially not attacking planes! Submarine submerge still an option.