This is definitely "The Damned" series by Alan Dean Foster. The first book being "A Call to Arms".
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 finds that humans are uniquely suited as allies, in that they have the ability to fight. They 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. Eventually Weave xenopsychologists determine that Earth's fragmented continents resulted in a species that evolved to fight, compete, and kill for dominance. They surmise that all humans are born hunters and killers, but that intelligence is starting to drive them toward the abhorrence of violence that other "civilized" species consider normal.
When Humans are unleashed upon the battlefield, all inhibitions against killing other humans are disregarded. Humans have no problem hunting and slaughtering non-humans, and some seem to enjoy it. This is appalling to the civilized races of the Weave, but they cannot deny the efficacy of human combat troops.
Humans have the potential to become fearsome allies for the Weave and are also physiologically immune to the Amplitur mind control abilities.
