Or British. Or Swiss. Or Spanish. Or Italian. Or Russian. The French got around quite a bit when it camr to stealing land.
Seriously, if you guys collectively were to defend against bretonnia, you would get nailed to the walls, and repeatedly prodded by lances. You SEVERELY underestimate bretonnia's capabilties and assume a doddery old halberdier could hold our keeps for several years whilst they futiley assault our walls with lances. In the event of war, hordes of peasants and knights would swarm into the south empire, and through dint of the fact that the southern armies are in the north, the northern armies are dead or in the north, and the dwarves dont like to get involved with domestic disputes, they could gain some serious ground and reach the reik with relative ease.
Of course we would respond by moving our armies south, but this would leave the armies guarding against chaos too weak, so just the southern provinces would return. And a few southern provinces, already weakened, against the bretonnian war machine would only go one way. Stalemate. Our only advantage is press ganging willing militia, but we all know the futility of two hand weapons against knights.
And all the while, bret's 'allies' would help them. Not willingly of course. Mercenaries in Marienburg, whether under command by the burgomeisters or not would start pillaging nordland for greed, revenge or just plain boredom. The border princes would take this historic oppurtunity for a land grab, and pillage parts of averland and stirland, further distracting from our abilty to cope. For every band of peasants that rightly revolt against the bretonnians ideals, another will turn to banditry, and further our woes (and theirs incidently, but thats another point). Even the dead may came to the party, which ends badly for all concerned, and may put an end to our silly war as they unite gainst a common foe. The worst (or best depending on your view) thing that could happen would be for chaos to invade, for no one is stupid enough to squabble amonmgst ourselves whilst barbarians invade in the north.
The again, no one is stupid enough to invade the empire, so the entire argument is academic. If Archaon's Chosen Cheese couldnt do it, what makes you think a load of dudes on ponies can?