Consider this:
IF the decision to go to war with the Zuma was passed by the Veinsormoot we would have D'hara, Barca and Terran on board ( since this scenario is in the future Barca has a tough military and D'hara has few food problems due to Barca's surplus).
Asylon would most likely get involved because of their good relations with Terran and the Veinsormoot.
Allison and Golden Farrow would get involved because of her dislike of Garret. (again, since this is the future we can assume GF has developed into a decently tough realm)
The other SA realms (Astrum, Morek, Corsanctum) would most likely get involved to support Allison.
The Lurias would join due to their religious quest to cleanse the monsters of Dwi. This would include PeL, LN, and the new realm in Shinnen.
This leaves Madina, Fissoa and Libero/ Summerdale without a decent reason to join but I imagine Madina would since they envision themselves as some sort of "free world" power and Libero and Summerdale would because, hey, why not, nothing better to do up there. And Fissoa would likely also end up sending troops just to not be left out lol
Now we have 14 realms...against the Zuma....
