Why 20%? Is there any reason for ZOI nations to fight other ZOI nations?

I'd imagine the biggest reason would be greed. Guilds will try to "control" as much land as possible. And depending on the guild, they might only be able to do it in one particular way (pvp fights or questing or money or numbers), so even if they already "control" one metro - they'll want more. This was true for castles in L2 and I'm sure at least some guilds will try and attempt it in Ashes. And it's way easier to destroy smth and then build it up with you at the helm rather than just overtake it (especially if you need to change the metro type).

So pretty much what Blood said.

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You want this one thrown into the Q&A randomizer?

My group is out of questions to ask and you already used yours.