Book #3 of the Damned series; by the author of the Pip and Flinx series, and many Star Trek books.
After millenia of relentless war, the union of alien races called the Weave was on the verge of winning a decisive victory--thanks to their new allies from Earth, who in a mere handful of centuries had proved masters of combat. But then the birdlike Wais scholar Lalelelang found disturbing evidence that the Humans might not adapt so easily to peace--that natural Human aggression would next be turned against the Weave, unless they were once again confined to fight among themselves.
When her field research revealed the existence of a secret group of powerfully telepathic Humans called the Core, it looked as if Lalelelang would be the first victim in a new war between the Humans and their allies. But just as her fate was sealed, a lone Core commander took a chance on her intelligence and compassion, gambling the fate of Humanity on the possibility that together, they could find an alternative to a galaxy-wide bloodbath . . .