[Taqwus]

Auto-retreat can also mean auto-death depending on what happened in neighboring provinces and how the attackers got there.
It would make a certain degree of sense to have a more detailed pursuit model comparing mobility, ranged firepower et al -- for instance, cases where one or both sides have flight available -- but the presence of magic really, really complicates things e.g. does a unit with intrinsic flight such as an Elemental Queen of Air have an advantage over a unit that could fly just because a handy air mage can cast Flight on it?