Collectively defend seems... odd. Perhaps collectively compete for control over would be more likely, unless there is some premise I'm missing. There are a few pirate groups who operate in the core, but if this is Game of Thrones with 'mechs, there will be competition. There may be an uneasy detente, but... that may change. Battle lines will be drawn.
As FedSuns, I expect us to be automatically aligned against the Combine and the Confederacy. The League contingent, being Leaguers, will be up to who knows what, because they're so bloody factionalized it's mad. The Commonwealth, the traditional ally to the Suns, will probably be our closest ally, but we'll see how that pans out.
Those would be the obvious, however, so I'm keeping my awareness open. The Lyran social generals occasionally get up to stupid things with their reliance on weight of metal, and they have their own factions. If I see a bunch of Skye natives, I'm polishing my rifle. Those guys never settle down.