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.