This is my interpretation of the rules & what I coded:

At the very start of the round, if any unit has the attachment canDefensiveRetreat = true, the defender gets a prompt if they want to retreat, and to which territory. They may only retreat to friendly/neutral territories, not an enemy territory, and they can't retreat to a territory with enemy units in it, unless only enemy air is present, which is non-blocking.

If the defender chooses not to retreat, then the battle proceeds as normal. If the defender chooses a territory to retreat to, then that territory is saved and stored for later. The attackers fire at all of the defending units. Any units that are doing a defensive retreat do not fire back. Casualties are removed, and then any retreating units retreat to the destination we stored earlier. Any defender units not capable of retreating remain and must fight to the end/get captured/get removed, whatever happens normally

EDIT: I was originally going to have a property that applies to all land units, but it turns out that in A&A North Africa not all land units can defensive retreat (the truck and the supply token can't) so it pretty much needs to be a unit attachment. This offers some variation as the mapmaker can select what units they want to be able to do a defender retreat

@LaFayette This game: https://forums.triplea-game.org/topic/4074/north-africa-triplea-module

Here is the rulebook I've been looking at, Pg. 20
https://renegadegamestudios.com/content/File Storage for site/Rulebooks/Axis Allies/A&A_NorthAfrica_Rulebook_v8_WEB_090324.pdf?srsltid=AfmBOopVwvgEyZeJnjYp_ruRU2jn9aTpuvxW6KJfxVfl6WqGoXWC8mog

@iratoric which North Africa game? AFAIK some of the variations are mutually exclusive. I think the first task it is to identify which variations should be plausibly supported. Once done, I would like a full spec identified. Following, we can then look at the plan for how that would be encoded into XML. Doing so iteratively and not all-at-once is perfectly fine. Though, I think we should know beforehand which retreat variations we are likely to want (and also in which maps).

@iratoric said:

Thanks everyone for your opinions. I have thought about this a lot, and it is probably best if we start with implementing one set of rules and branch out from there. Otherwise there are simply too many possible combinations to do all at once. Since the North Africa game is the “official” ruleset and an already existing game that needs this rule, I think that whatever I implement should support that first. I will code it in such a way that it still leaves room for more variations later. Thoughts?

I instead think that the baseline should be some version of what you/we can devise as the simplest retreat system, have that done and tested, then adding properties and/or options for each element adding complexity.

For example, in the basic system you retreat to the same zone. Next step, with property, is retreating to an other zone. Next step, with property, is limiting retreat only to zones not under attack by land units. And so on.