Ttyjukl

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.

That would be the Damned trilogy by Alan Dean Foster, consisting of A Call to Arms, The False Mirror, and The Spoils of War.

Two major alien civilizations, the Amplitur (a squid-like species with telepathic and mind-controlling abilities - which they couch as "suggestions") and The Weave (a confederacy of more or less equal species), have been fighting a war for several millennia. The Amplitur are attempting to join all sentient species in what they call the "Purpose", an alliance which they "guide" to some unknown (even to them) end. The Weave is a group of species allied in opposition to the Purpose.

Most of the fighting takes place on planetary surfaces, and is relatively restrained in terms of destruction, the purpose of the war being to convince and control one's opponents rather than destroy them. However, most sentient species in the galaxy have evolved to be incapable of committing violence against other sentients (violence of any sort being most un-civilized, but against another sentient being a [literally] unthinkable crime), which leaves a shortage of warriors on both sides. The Amplitur with their mind-controlling abilities and therefore ease with which they control conquered populations have gradually been pushing The Weave back for centuries and seem to be on track for final victory.

On a mission to find new resources and allies, a Weave scout ship discovers Earth circa late 20th/early 21st century AD and find that humans are uniquely suited as allies, in that they have the ability to fight, first of all; are adaptable to a wide variety of environments, have few (or sometimes no) compunctions regarding war (humans having been fighting each other for all of their recorded history), and above all seem even more enthusiastic when their aggression is focused on non-humans.

Humans have the potential to become fearsome allies for the Weave and are also physiologically immune to the Amplitur mind control abilities.