All Borg controlled planets, fleets and infrastructure circa TNG and Voyager eras is teleported into Empire controlled Star Wars space.
The Emperor, Vader and Rebels call a ceasefire. The Deathstar 1 is complete. They are aware a threat is invading but neither side is given much more than a cursory knowledge of the other to start and they do not have access to each other's locations to start.
NO TIME TRAVEL ALLOWED
How does this play out over the following years?