Exodus: Starfire, Book 4 (hardcover) Steve White & Shirley Meier
Once before, the sentient races in the known part of the galaxy -- humans, Orions, Ophiuchi and Gorm -- had united to defeat alien invaders. The "bugs" were as incomprehensibly alien as they were revoltingly evil, using all other living things, intelligent or not, as food, and they had been defeated at a terrible cost. Decades have since passed and the gallant warriors of the battle against the bugs have grown old, while new generations have grown complacent...dangerously so.
Long ago, much of the population of an entire planet had built a huge fleet of ships, each ship larger than a city, and fled their world before its sun went nova. Those slower-than-light ships traversed many light years, and have now arrived at the world they intend to make their new home. They regard the fact that the planet is already colonized by humans as a mere inconvenience, the more so since their mode of communication is so different from anything humans use that they do not consider humans and their allies to be truly intelligent. And the arriving aliens know -- or, at least, they believe -- that when they die they will be reincarnated, so they do not hesitate to attack humans and their allies with suicidal fury.
This time, the intelligent races of the old alliance will not have to worry about becoming an invader's meal -- but that will be small comfort if the invaders decide that genocide is justified for their own survival...