Wiping out the Roman Empire would probably prevent the US from ever existing, at least as we know it. So the marines wouldn't exist to wipe out the Roman Empire, etc. etc.
I don't see how a serious movie could be made out of that anyway... what possible motivation could the marines have for wiping out the ENTIRE Roman Empire vs simply finding themselves a nice out-of-the-way area to hole up and live "happily" ever after?
Buzz Killington, signing off.