I recall there were two warring alien empires, one of which was led by a race that could telepathically inflict pain on others, but when they tried to do it to humans, it would backfire. something about human's animal instincts being much closer to the surface than the other species.
Humans made the most aggressive and dangerous fighters, so both empires tried to reruit humanity, since the other races involved in the war were fairly monolithic and sided all with one empire or the other. Humanity was all splintered though.
The empire led by the telepathic race claimed that everyone in it was equal but in reality the telepathic race was in charge.