The lore is contradictory. In the SP campaign there is a bit where you're in Sigma-13 and they drop the line about GMG and RM and 80-yr-war, then the next chapter shows RM battelships and cruisers and gunboats stacked on each other in New Berlin and Frankfurt, and they have already invaded the Sigmas and wiped out the Kusari fleet in Honshu. Then the battleship rumor cards for missions talk about RM being broken. Makes no sense.
Kusari has gotten its ass kicked multiple times over, and hasn't ever won anything that I know of. They also have the fewest stationary battleships--Kyushu, Hokkaido, Shikoku, and Hiroshima. They have to be at the bottom.
Lore says Rheinland is a spent force, crippled with debt and that can only have gotten worse after the SP campaign. They currently do have a large number of ships--Dresden, Frankfurt, Hamburg, Munich, New Berlin, Omega-15, Stuttgart, Braunschweig, and Bremen--but they are all in disrepair and undermanned, and some of them cant even move anymore. They are a broken force.
Bretonia was not directly beaten in the campaign or any other time that I know of. They have at least 10 stationary battleships that I can think of--Cambridge, Dublin, Dundee, Leeds x2, New London, Newcastle x3, and Salisbury--and there are the two Exile ships there too. There's no real evidence to suggest they are a broken force, tired but not broken.
Liberty's strength is unknown. They were able to hold back Rheinland, but so was Bretonia. They have th at least 11 stationary battleships--California, Colorado, Illinois, Minnesota, New York, New Hampshire, Ontario, Quebec x2, Texas, and Virginia--and all of them are in good repair. They have the most advanced technology in the private sector too.
Gallia's fleet of stationary ships is larger than Liberty's. I don't think it's larger than Liberty plus Bretonia.
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Kusari is effectively out of it already. They could rejoin later for some purpose, but as of now they are off the board.
Its not yet know what will happen with Rheinland. They do not have direct contact so Gallia would be better off to just throw money at Rheinland and pay them to sit it out, and then push them over after the other houses have fallen and nobody is left to join with RM. Dunno if RM will do this but that is what Gallia should try.
If/when Liberty joins with Bretonia, the two of them will be able to push the Gallic invasion force back into Orkney. The longer they wait the harder it will be. If Liberty doesn't join fully with Bretonia, the Gallics would be able to beat them both individually. Maybe at that point the Kusari and RM forces could join up and get things moving enough to beat the stretched forces before they recovered. Dunno. Better for everybody if Bret and Liberty kick Gallic butt sooner rather than later.