Did the Norse or Saxon rulers of England in 1066 have landsknechts, cannon, handguns, (mercenary) pikemen, cuirassiers and generals who knew how to use them?
There's a reason why the medieval Knight lost his primacy on the battlefield by the time the Renaissance rolled around - as aptly displayed when I went to visit the Royal Ontario Museum on my recent trip to the GTA.
(As an aside, I was inordinately excited seeing cuirassier armour - yay, Pistoliers! - and landsknecht armour and Great swords... even including the wiggly sword used by the Empire Greatsword Champion! That is, when I wasn't thinking how great it would be to see Cathay done in WFRP, Warmaster or WFB while wanderign their Chinese exhibits)
The only potential ruler of Bretonnia who'd even consider full-scale war with the Empire would be Mallobaude (the Black Knight of Mousillon, as described in Barony of the Damned) and he couldn't give a whit about the Lady of the Lake's chivalry - nor is he adverse to hiring vampires, necromancers and black powder-toting mercenaries - and if he succeeded in conquering Bretonnia he would be a very serious concern for Marienburg, Bilbali and the Empire.
In fact, there'd probably be the equivalent of the kind of Crusade which kicked the Arabians out of Estalia - and they weren't necessarily evil (though Jaffar got a lot of bad press) - drawing the Tilean and Estalian realms and the Empire into the Kingdom... who'd probably try to divvy up the spoils while they were at it! I can even imagine L'Anguille serving as the equivalent of Magritta - the last unconquered city used as a rallying point for the Cruaders. (It's already defended by the Brethren of the Lighthouse, plus the Elftown could possibly do something to keep Tor Alessi from falling to darkness. Plus, it's handily close to Couronne, too!)
Still, better that than leave Mallobaude on the throne.
Gary