A really interesting concept to me. Backstory would be important, I think. But you could do it CO-OP style with plenty of civilians around that join groups with EAST if PLAYER is detected. Scripting might be easier if there were some already built in components via DIALOGs, perhaps ideally in the hands of one operator-type, who is a part of the action, and subject to being smote (smited, smoted, what's the appropriate spelling for this particular tense of smite?).
At any rate, if you do something like this, I'm thinking a massive, epic-style feel with multiple objectives, vast hordes of potential enemies, and built-in transportation options at various locations. You might even require a code-line before a PLAYER can operate the vehicles. Also, you might build-in a random selection of one or more objectives during the mission, so that no mission is every the same--even if it's 1 objective per mission or 3+ objectives per mission.
At any rate, I think the idea is pretty cool.