It's hard to see how the Cardassian Union can ever be a significant power again. I doubt Cardassia Prime would be occupied but there are sure to be strict limits on their military. They might be allowed powerful defensive systems like orbital weapons platforms but not much in the way of assets like battleships allowing them to project force outside their borders.
One thing is for sure: It will be very difficult for the three principle allies to hammer out a treaty which will be acceptable to all of them. For instance, I could see the Klingons and the Romulans insisting that Cardassia not be allowed to join the Federation - at least not without major concessions to them.
One thing is for sure: It will be very difficult for the three principle allies to hammer out a treaty which will be acceptable to all of them. For instance, I could see the Klingons and the Romulans insisting that Cardassia not be allowed to join the Federation - at least not without major concessions to them.