People allied with the invaders because to not do so was death. The realms that did not ally with an invader faction were mostly destroyed. Riombara and Thlamarkin survived quite literally by the skin of their teeth, and both realms could have fallen but did not because invaders chose to pull punches (or went inactive at a bad time, maybe). Both realms lost battles in their *last* region at which point the invaders oddly did not begin a TO, or abandoned one in progress (I think thats what happened in Unger). The realms that weathered the storm best were either in the undead corner of the map, where the local invaders were inexplicably merciful compared to their southern and western counterparts, or were firmly allied to one faction or another. The lesson drawn I would say is that discretion is the better part of valor. Honestly if all the human realms had resisted to the end, the invaders would have probably conquered the map. It's not like human armies were capable of stopping them. Realms survived by taking sides and avoiding the wrath of the invaders closest to them.
I think we needed to have allied with each other and faced the invaders much earlier in the war. Once they had claimed regions easily off realms like us in DoA they had that much more scope to recruit armies. We were told that the way to fight them was not necessarily with our own armies, but it always seemed to me that very few characters were willing to co-operate to work out what the other means may have been.
