Yeah, sounds annoying. Honestly whenever you have a post-apocalyptic novel and the hero is a military veteran or reservist or what-have-you in a town full of civilians, it's a red flag. I learned that from Under the Dome. Why oh why couldn't the protagonist just be a short order cook? Even though Battlestar Galactica doesn't bother me at all, because there an entire ship with a standing hierarchy survives, you don't just get one guy who's like "well, I clearly have ALL the authority in this situation."

No, you certainly can't, and to some extent I think it would have been foolish for the former European colonies to try. But of course, they became independent in a globalized world with a globalized economy, not the world of the Roman empire.