And regardless of fictional setting, militaries should have at least some basic understanding of strategy and tactics. But then we get the rule of dumb.
I think it's called the NPC syndrom. If NPC were actually competent and could get their sh*t together on their own, it would probably a boring game. It would probably play out something like this:
Hackett: "Oh Shepard, good to see you. We have already built the crubcile after sucessfully forming an aliance with the Asari, Turians and Krogans. Also the Quarians were able to negotiate with the Geth peacefully after they let go of their foolish, genocidal plans. We are just docking the crubcile to the citadel this very moment, while the Reapers are still busy invading the batarians."
Shepard: "Oh good. What can I do?"
Hackett:" I don't know. Go and guard this door there I suppose."