Here are a few possibilities, in increasing order of awesome:
1. Bretonia becomes puppet state, except for a heavily fortified New Cambria, New Anglia, (possibly Dublin with molly assistance), and planet Harris, since everyone seems to have forgotten about it, BAF become guerillas, Privateers launch raids on Gallic shipping to supplement Bretonian economy. Liberty fortifies and prepares for the next Gallic assault
2. Bretonia becomes a puppet state as before, Liberty ceases war with Rheinland and sends aid, smuggles refugees and goods to Bretonia, while fortifying for the next Gallic assault.
3. Bretonia becomes a puppet state as before, but has time to build and launch This. (replacing part of the Liberty fleet with Bretonian ones, as Liberty offers assistance). A large portion of Bretonian population escapes and sets up a base in orbit of Planet New Cambria (assuming there is one). Bretonia gradually rebuilds fleet by sending swarms of privateers to cripple and loot Gallic shipping.
4. Bretonia launches This, all Bretonian systems are captured and Bretonia basically becomes a giant nomadic (not in the alien sense) fleet, staying one step ahead of the Gallic forces as it trundles across Sirius
5. Bretonia launches This, sneaks it and the remnants of its fleet into New Paris, destroys the jumpgates into the system, invades Planet New Paris with the swarms of troops aboard, captures and subsequently tazer-flogs the King of Gallia to death, and over the next few years, conquers Gallia and moves back to reclaim Bretonia, while Liberty and Rheinland move in to assist.