The series involves two groups of aliens who have been fighting an interstellar war for many years before the first book. The war is started by a race of jellyfish-like telepaths who have used their powers to control several species which they use as soldiers. The jellyfish aliens have declared a holy war against all sapient vertebrates. The only species on their side that aren't being actively controlled through their telepathy are invertebrate species who have joined the holy war.
In this series, humans are immune to telepathy--what's more, a completely unconscious reflex tends to destroy the mind of anything even attempting to affect their minds. One of the central themes of the series is how well suited humanity is to warfare, and all the sacrifices forced on humanity as they become nothing but soldiers fighting against the jellyfish, losing almost all culture not in some way related to being soldiers, as the other races in their alliance take over the other roles.
Other species I recall include a bird race that is gifted at linguistics but can be sent into shock by even seeing a low quality recording of a human walking around a room. They just cannot handle the hide or flee reflexes triggered by seeing such an obvious predator. Another race is a crab like group that is so antisocial they all keep suicide buttons at hand when working with other races, and only work to help the human alliance's side because the jellyfish won't leave them alone. There is a race that bears a faint resemblance to large rats who were previously the best soldiers on their side, and another renowned for their intellect.