There's a trope in science fiction in which there's a race/species (usually the protagonists of the story that achieves inter galactic travel). They head out in to the wider galaxy (sometimes galaxies depending on setting), they then encounter many alien species who for various reasons are often at war with each other but this race acts as some kind of inter-species peacekeeper, a mutual ally for all those various races to form around and stop fighting each other.
The obvious examples are humans in Star Trek – they first met Vulcans, then Klingons and then others. The Vulcan and Andorians hate each other but but with humanity's introduction and a long time frame, Starfleet and then the Federation arose, with Vulcans, Humans and Andorians.
What is the earliest example of this trope in which a species that has helped to bring a divided galaxy together to bring about some level of peace and cooperation?