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So my dad and I were talking about books last night, and he mentioned a science fiction book series that he read when he was younger than me. I'm currently a young adult, not even 20. It had to have been published before the 2000s then. He talked about how much he really liked it and how he wished he could find them again. I want to do some hunting on the internet to see if I can find them and surprise him. Okay, this is all the info I know and it's unfortunately not very much. He doesn't remember any of the character's names, the author, or cover art.
It was a series, not a stand alone novel
The plot involves two alien factions fighting against each other "Kind of like the Federation and Dominion," my dad said.
Humans aren't initially in the story, but they come in later when one of the factions employs the human's help in fighting
At least one of the factions doesn't like fighting, but they do it cause they have to (like the Federation from Star Trek). Because of this, when the faction finds Earth, they are surprised to find a myriad of governments and the humans seem to like fighting.
The humans fight on one side, they kick all kinds of butt, but after the war the aliens are mystified. "Well, what do we do with them now?" Sort of a thing.
Earth at the time of this novel was probably in the 80s or 90s, I'm wagering.
Any help or guesses would be appreciated. It would be great if I could surprise my dad with a copy of the first book in the series.
EDIT: It is indeed The Damned Trilogy by Alan Dean Foster my dad was looking for. I was hoping it would be in my local library system, but I did not see it. So I showed him the cover art and he instantly recognized it. I'll probably buy it for him for his birthday. Thank you to everyone that helped