Well they do have an empire to fall back on, although the loss of life, culture, and production would be crippling. I expect various colonial governors and military officials would try to seize control, and a big ol' civil war would break out. Also, The Klingons would no doubt be tempted to take advantage of the situation, too.
Come to think of it, that might tie nicely into the Star Trek MMO timeline: if the Klingons did try to make a grab for Romulan space, the Federation would no doubt oppose it, and the khitomer Accords would collapse (again.) This would fit the situation that is supposedly going to exist at the outset of the game.