To make the Romulans more sinister (as if they aren't sinister enough) they set up the whole ecological mess in the first place.
After they secretly emplaced the weather control equipment, they used the equipment to create an ecological disaster on a scale unseen before. You know the stuff, floods, tidal waves, typhoons, etc...
After a few months the Romulans arrive and "volunteer" to help and quickly the ecological mess is cleaned up by simply turning off and reversing the weather control gear. All very messianic and soon the whole world believes that the Romulans are their saviours.
Now firmly in control the Romulans begin to take what the world has to offer. Dilithium and other raw materials (of course the expenditure has to be worth it). Of course the inevitable resistance movement arises and they uncover part of the weather control system and some are killed leading to one brave soul stowing away on a freighter and gets himself smuggled into Federation space...the rest of course is up to you.
Arise, arise, Riders of Theoden!
Fell deed awake: fire and slaughter!
Spear shall be shaken, shields be splintered,
a sword-day, a red-day, ere the sun rises!
Ride now, ride now! Ride to Gondor!
Theoden King: The Return of the King