So they should set it in the future? If there's a CS book, doesn't that set the state of the world at a certain date?
I dont know if an adventure with no fixed time line is set in the future or the past.
If you really really need a city that has been invaded by a foreign army then I can think of plenty of examples you could use, like one of the Dales by the Zhents or Neverwinter by the Red Wizards. Remember that the Realms is not a peaceful place.