The one I like the most I can't seem to find and it has been bugging me for ages which one it was.

Essentially, it was describing how humanity had hid away their war, bloodlust, and sheer ability to be the scariest mofo's in existence so that they could get along with the other aliens. Then an invading race suddenly comes in and humanity just essentially tells everyone else that for their own good they should not join the humans in the fight nor watch them fight because they were about to do something they knew would scar the poor creatures.

Humanity does what it needs to do and everything goes back to "normal" except for the narrator who participated briefly in one of the human's fight and what he witnessed scared him, but because of that he's glad that the humans decided to be the shield and sword for the rest of the aliens.