I'm just wondering about how we view the strategies, capabilities and character of the various races so propose this exercise to sate my curiosity and to hopefully pique your own...
What if the Romulans had been influenced by the founders to invade the Cardassian Union instead of the Klingons doing so?
How do we think the other powers would react? What strategies would the Romulans use, and what measure of success would they have?
I remember Klingons ('Dogs of War' maybe?) talking about Cardassians using sensor shadows and warp ghosts etc, and about the Klingons deploying mines and so forth to fortify captured systems and that got me thinking how Romulans might act in such a situation if they were the aggressors...
In my opinion the Romulans are adept at sudden pre-emptive strikes as well as sowing dissent amongst their opponents, they also seem to be casually brutal at times as well as compassionate ('Birthright' etc). I was thinking that the Romulans would invade Cardassia in a similar way to the Klingons but without the pit stop at Deep Space Nine. The factors I can't really deduce relate to how the other powers would react and in turn what the Romulans would do so I was thinking...
Maybe the Romulans create a series of diversions to occupy the UFP and Klingons perhaps a series of accidents, false flag incidents and assassinations with the intent of the UFP and KE fracturing their alliance and bickering, allotting blame to each other. Meanwhile the RSE invades the CU rapidly and brutally employing weapons such as mines and deployable orbital weapons platforms possibly even using biogenic or similar weapons to try to cow the Cardassians into a quick surrender or negotiation leaving the Romulans a hefty area of the CU.
So the outcome would be the Cardassians retaining their core world and several systems whilst the Romulans occupy a large part of the now former Cardassian Union appointing overseers from the Romulan Navy as well as indigenous natives.
The Romulans would need to be in a position of strength lest the Klingons exploit the distraction and of course because the Romulans do appear to be a paranoid people anyway. I don't think the Romulans would have the time or inclination to conquer Cardassia itself but other views are welcome.



