Exactly. And as Union it should be vice versa. It's ACW
However, i know how the engine works, and therefor my question above. Okay, i see it concerns majorily Mexico and British-Canada, but they are also the focus in my fear to get a pretty ahistorical war in ACW.
If we would have the modding abilities which we have with the M2TW (or RTW) engine, one could make a historical accurate ACW mod regarding AI interacting simply with map-design (ie. impassable areas and/or immobile/mobile (traits, script) armies and/or drastic recruitment limitations), while the AI-diplomacy/CAI behaviour-modding would be limited though in this older engine.
For the latter case (diplomacy, strategical AI aka CAI), ETW engine is far more advanced ... but indeed, this might not be enough to make "idle" AI there, it could also need limiting building and recruitment capabilities to keep the 'sidenations' really idle (towards CSA and Union). But ie. what is, if Union declares (idiotically as the AI is) war to British-Canada while it fights CSA and therefor has a loosing two-front-war (or just CSA vs. Mexico)?
Next issue is the victory conditions, and playable factions question ... .
Whatsoever, as you can see, i personally would prefer a mod design here, that offers an ACW conflict alone, in principle a combi of M2TW map-modding with a huge detailled NA map extract with only Confederates and Union - plus the advanced ETW engine would be needed for this what i would like to create as ACW. But that's me, i'm just against the TW-What-If approach, and in all my modding efforts, i tried to limit ahistorical chaos (game/immersion-breaking for me) through campaign balancing. Of course AI never plays historically (if the mod is not a complete script-steered one), but the player can, and therefor steers the game at least to a degree, the ahistorical ways it gets then inevitably are not so immanent with a world map and huge timescale, but for the ACW conflict i have just bigger issues to experience the game-breaker-effect.
EDIT
Perhaps a suggestion:
a) ACW campaign 1, ETW vanilla map - offers a What-if ACW design.
b) ACW campaign 2, ETW/Warpath map - offers a (as far as possible) die-hard ACW conflict focused design.