That’s like The Damned series by Alan Dean Foster.
Basically alien societies naturally formed without the type of infighting and barbarism humans started with, so the entire idea of war to them was extremely foreign and uncomfortable until they were attacked. They formed an alliance to push back the empire trying to conquer them, but they lacked the will or martial ingenuity to really hold the empire back. Then they come across humans, and the first (random) human they come across accidentally severely injures their ambassador. So they offer humans heaps of technology and resources for soldiers. They don’t offer membership (because we’re absolute nightmares) if I remember correctly, but Earth is fine with that. People volunteer in droves to see the galaxy and fight in wars where they are so overpowered it feels like they put in a cheat code.
The series is about… well, what happens to our society when our main export is unstoppable death and destruction. Hence “The Damned.”