I totally agree with Theripontigonius, the Empire war machine is built for large combats against large armies, they have such a mixed bag of units that you can pick anything in the Bret army and have a defence against it. We have wandered into the realm of using history to explain fantasy but if ever there was an example of Knights getting boned, just look at Agincourt, loads of absolutely insanely bad ass knights in the best and latest war gear in the medieval world at that time, bogged down and killed by lightly armoured mental English Longbowmen wielding hammers.
To sum up Brets are just too one sided to win against the Empire. Coupled with the fact that state troops are vastly superior to the inbred peasants, and when all the knights are dead (which will happen as they are nobility so there aren't that many of them), the Empire will win.
It is said that the guttural sound of Dwarfs bellowing Grungni’s name is enough to make an Elf’s knees knock and a Goblin turn a sickly shade of yellow!