Such alliances were necessary. If every realm had been defiant to the last and refused to make any alliances with the invaders, there wouldn't be any realms left. The invaders would have been forced to conquer every realm they encountered instead of what happened, which is that they destroyed some but allied with others and then fought each other to an extent, occasionally on behalf of their allies (or at least in ways that served to protect them).
In my opinion, the invaders were eminently capable of conquering the entire island. No human armies could have stood against them, even if every realm had willingly allied and devoted every effort to attacking the invaders. Does anyone seriously think that Mesh or Heen could have been saved? I seem to recall hearing about 100k CS concentrations of daimons at some points during the invasion. Their massive armies ensured fast takeovers, they had effectively limitless resources (from everything I was able to observe), and their units were nearly unbeatable. Sure, some monster and undead armies were defeated by humans alone at times, but the invaders never appeared to be scraping the bottom of the barrel recruitment-wise, and experience showed that even if you killed off the commanders, the GMs were allowed to create more characters. Each character was an army unto himself, capable of challenging all but the largest human forces. And they had quite a few characters.
