As for "who would win", it's difficult to consider Gridania as easily as the other two nations. It is literally a theocracy that's "religion" is really /real/, the other two nations are not built on a magical superpower. Being smote by the Elementals is a real danger, not an abstraction, and Gridania's existence is so tied to the Wood that they're really the same thing at this point. So attempting to destroy Gridania is the same as attempting to destroy the Wood.Â
If we're reducing the argument to "which side can obliterate the other", then the way to beat Gridania is by convincing the Wood that it doesn't want it anymore (which may be impossible without just destroying the Wood entirely). This would be a very difficult thing to do, and maybe impossible, the Wood is huge, much larger than what is depicted in game. The Calamity was able to burn some of it, and I doubt whatever fires Ul'dah or Limsa tried to set compares to Bahamut's fury. So Gridania may be impossible to destroy without Gridanian resistance, and with the Gridanians working to thwart your efforts it's definitely impossible without leveraging something like Allagan super-weapons.
But it also cant really destroy anyone else, so it can't "win" either. Gridania is only interested in external threats. They're largely self-sufficient and don't really even need trade to sustain themselves, because of that their military is structured mostly for defensive purposes. I can't see the might of Gridania being able to destroy Ul'dah or Limsa Lominsa. Ul'dah or Limsa Lominsa could destroy /each other/, and then be in a stalemate with Gridania, but that's where it'd pretty much stop.
If we're reducing the argument to "which side can obliterate the other", then the way to beat Gridania is by convincing the Wood that it doesn't want it anymore (which may be impossible without just destroying the Wood entirely). This would be a very difficult thing to do, and maybe impossible, the Wood is huge, much larger than what is depicted in game. The Calamity was able to burn some of it, and I doubt whatever fires Ul'dah or Limsa tried to set compares to Bahamut's fury. So Gridania may be impossible to destroy without Gridanian resistance, and with the Gridanians working to thwart your efforts it's definitely impossible without leveraging something like Allagan super-weapons.
But it also cant really destroy anyone else, so it can't "win" either. Gridania is only interested in external threats. They're largely self-sufficient and don't really even need trade to sustain themselves, because of that their military is structured mostly for defensive purposes. I can't see the might of Gridania being able to destroy Ul'dah or Limsa Lominsa. Ul'dah or Limsa Lominsa could destroy /each other/, and then be in a stalemate with Gridania, but that's where it'd pretty much stop.