From what I understand, the Eorzean cities have almost always been at war with each other, and with the tribes they share their lands with. It's only recently they've come together, because they've found a common goal: protecting the realm they all call home.

It's a fragile thing, built primarily on trade and a desire to end bloodshed. That isn't to say they're all friends, far from it. Decades of war and all that comes of it can't be healed overnight.