The rebellion ran away from the Empire, never really facing them head on.. Well not until their final battle which as you might remember was a great loss for the Republic... I still don't understand why the Empire didn't just finish them off after the Death Star was destroyed but that wouldn't really be a good ending now would it be?
Anyways the Rebels never really tried they fight they more the kind of people that took out vital places where the Empire was weak. Something that would really hurt them but never really destroy them. If they had a chance in hell against the Empire they would have send their fleets to Coruscant and leveled the Senate building.