I played the invasion aspect a bit more underhandedly as Mirror infitrators got into secondary positions of power and quietly diverted resources to construct a "gate generator" to weaken the fabric of space to allow the Imperials to invade with a fleet and begin coneqest of the alpha quadrant
As this was during the Dominion war, Starfleet and the Dominion had thought the otherside had diverted resources for thier benefit. While starfleet sent the players to investigate, the dominion sent a warfleet
And got pranged hard by the imperials coming through, as with anything, they had concetrated on building cutting edge warships and worshipped Power.
The end restul being a lot fo scrap for the Federation to pick over and reverse enginner
A brave little theory, and actually quite coherent for a system of five or seven dimensions -- if only we lived in one.
Academician Prokhor Zakharov, "Now We Are Alone"