Being a GM controlled faction doesn't mean they are untouchable. Sure, they make surprising shows of force here and there...but they've done less damage than most realms against one another. A (good) GM is supposed to make the interaction engaging and fun...so far it sounds like most of that has been RP. That doesn't mean there isn't tactical fun to be had.
Every realm surrounding the Zuma could be blighted and there would still be plenty of room for every character displaced. The only reason people haven't prodded the Zuma is fear of losing fancy titles and perceived ownership of whatever region they squeeze gold out of.
One of the most fun times I've had on Dwilight was the initial colonization of the Divides in the creation of Swordfell. The hopelessness of oppressive starvation, horrible roads, and near constant monster-sprouts was !FUN!. The pressure of a real challenge is engaging, and the more engaged people are the more they interact with one another.
The mere idea that a coalition of realms could go against the Zuma make my pants sticky. I can't be the only one that would burn half of Dwilight for the joy of it. One day....one day...
